Personally, I believe virtually any team is viable, doubly so when made of characters that came with the adventure path your are playing. That being said, you will have some challenges. Merisiel and Jirelle have a good bit of overlap in what they do (dexterity and finesse weapons) and you won't have a viable consistent means to heal.
But, if you keep those things in mind, you can be successful. Be careful about discarding cards and try your best to acquire some boons that can sure up your weaknesses (like emerald of dexterity). I'd give it a try and if it goes horribly, change your character.
Just with the basic box? Let's consider your choices briefly.
Alahazra:
+Divine spells (reliable healing, plus can retrieve Cure from her
discard to improve reliability further!)
+Best Charisma, which will help with the Diplomacy problem.
+Divination with a recharge cost. She helps herself and allies peek at upcoming encounters.
-VERY bad if forced into a martial situation, and/or vs. barriers, because she d4s both Strength and Dexterity.
Lem:
+Arcane and divine spells (reliable healing)
+Good Charisma with Diplomacy
+Supports himself and friendlies at his location
-Can be hard to decide whatall to give him, due to his diverse options.
Lirianne:
+Guns, guns, guns.
+Supports friendlies at other locations
+Best Wisdom, giving you a second option vs. ships (all of which are Wisdom / Survival along with something else)
+Has Fortitude
-Her stat spread is VERY similar to Jirelle's, just reversed, so there's functional redundancy there.
Seltyiel:
+Arcane / martial hybrid
+Gets another chance to recharge discarded spells
-Doesn't stand out in any stats, and (like Merisiel) d4s Wisdom.
Valeros:
+Won't have gear competition for non-finesse weapons or heavy armour.
+Passively supports friendlies at his location.
+Has Diplomacy and Fortitude.
-Doesn't use magic, and (like Jirelle) d4s Intelligence.
----
That all said, try it with Seltyiel if you'd rather use him. :)
I share the opinion that any party is viable. So more than power synergy or fully-covered stats, the thing I try most to keep track of when creating a party is potential for loot disputes, especially when playing with kids.
Jirelle and Merisiel both use Finesse weapons best so they will be fighting over them, which alone is probably fine. But most of those Finesse weapons are one-handed melee weapons, which is something Seltyiel would be looking for as well. And Lem prefers Finesse too if you switched to him instead (although when playing as support it doesn't matter as much). So every time you see a Finesse weapon you will have to decide which of the three of you get this one, which will make it much slower to progress each character than if they had sole access to their craved cards. And I don't know about you, but if they were my kids I would just end up with a deck of hand-me-downs.
Another side effect is that while you all rush to grab one subsection of weapons, through your entire campaign any time you encounter a Ranged weapon, Two-handed Melee weapon, Divine spell, or non-Attack Arcane spell, no one will really care. That might dull things a bit.
A couple points:
1. If you're trying to get your kids interested in the game, it may be wise to pick a strongly supporting character like Lem, Lirianne, or Valeros (or Alhazhra to a lesser extent). This is not so much to make a balanced party as much as it is to let your kids really shine. IF you're spending your time supporting them, they will get more opportunities to get the loot or beat the bad guys, which may get them more engaged. The reason I consider Alhazhra as a lesser option than the other two I listed is that her support is more proactive, so there is some risk that your kids may feel you're taking control of the game if you use it too much. the support abilities of the other 3 are much more subtle.
2. Several people here have pointed out that both of your kids have taken Finesse melee users. I do want to point out, though, that Merisiel, despite that power, is still really good with ranged weapons. She's losing +1 to melee, but often will get weapons with bigger dice, especially once she has proficiency. So, while it looks like there's a lot of overlap there, keep in mind that Meri has a lot of flexibility here. (That being said, adding Lem definitely does risk overload, since both Lem and Jirelle more strongly favor Finesse over ranged.)
3. While there are some early examples of opponents with secondary skills like Charisma/Diplomacy, those tend to be the exception, not the rule. And outside the base set adventure, it is often not charisma, so it isn't absolutely required that you have someone with good Cha (though it's certainly not bad). The best secondary skills in S&S are Survival, Survival, Survival, and Fortitude, with Craft a distant third. (yes survival is that important, ime, which is why Jirelle is quite good here).
4. Upsides and Downsides for each character:
Selytiel
Upside: Gives you Arcane, fairly complicated playstyle
Downside: Tends toward being selfish, not good at helping others. Considered underpowered, especially early, by many players. Bad Wisdom score
Alahazhra
Upside: Can really help your allies with scouting abilities, great Charisma, gives Divine (and with that healing) which will be really nice if your kids get too aggressive.
Downside: Can become something of a controller if you scout too aggressively, which turns off some players. Can have trouble early before she gets any sphere spells with unexpected combats. Not many skill points.
Lem:
Upside: Support ability is probably the best in the game. Both arcane and divine skill is wonderful, even with the low check value. Ability to choose your favored card means you can specialize a lot easier. He cycles his deck lightning fast, and the ability to shuffle recharged cards means that you will often be able to use good spells many times in a game.
Downside: Can sometimes struggle on his own. Combat is never better than middling, even with support power being usable on self. Will often not be able to do much on his own turn, due to small hand size from support. Low number of spells is going to lead to some very tough decisions.
Lirianne:
Upside: Great at combat, in this party will probably consistently be the best fighter on the team. Ability to support combat checks will help allies succeed, while cycling your deck. High Wisdom will be useful.
Downside: Will take the best Ranged weapons, meaning Meri and Jirelle will likely end up fighting over finesse and not be able to fall back on ranged. Hand size 4 means you will not be able to do much on the average turn. Low-combat scenarios can be very rough.
Valeros:
Upside: Great at combat, and good support for allies in combat. Solid Fortitude, and has good enough diplomacy early on.
Downside: 4 card hand size means you will often not be doing much. Terrible at cycling his deck without combat.
In the end, like others I would recommend picking something that you will enjoy playing. However, if you don't have a strong opinion, and just an initial lean, I'd recommend steering towards a more supporting character, to try and give your kids more spotlight. Hope that helps!
Jirelle, Merisiel, and Seltyiel are actually 3/4 of my Skull and Shackles party. As far as combat and weapons, it's actually worked out well:
Jirelle has often had a gun on hand, plus finesse weapons.
Merisiel has moved almost exclusively to guns and other ranged weapons.
Seltyiel early on favored the icy boarding pike- he couldn't use his ability to combine it with a spell, but he was usually rolling 2d8+1d4+4, so it didn't matter. From back half of deck 2 and into the start of deck 3, he's been finding more one-handed non-finesse weapons, although he's also taken some finesse, without impacting Jirelle.
So, my experience is there are enough weapons to go around. Other party deficiencies might be harder to overcome, but allies and blessings can help (and sometimes items- Merisiel does well with the emerald of dexterity).
(my caveat is that the fourth member of the group is Lem, and he can go quite a ways to making up for the short comings of others).
S&S is designed around having lots of Dexterity-based and Finesse-based weapons so I wouldn't be too worried about the overlap.