As for a weapon, may I suggest a longspear.
You don't want to actually attack with it. Instead you want to use the Aid Another action to try to help Pumpkin the Tiger score hits. Since a longspear is a reach weapon you can stand behind pumpkin while using it. Your chance to hit will be pretty bad, but you only have to hit an AC 10 to use aid another, and this will grant Pumpkin either a +2 on his next to-hit roll or a +2 to his AC (your choice).
The problem with both acid splash and any ranged weapon is that once your enemies are in melee with your friends you will not only get a -4 penalty to hit because they are in melee but you will almost always also get a -4 to hit because they have cover (provided by your friend who is in the way). The sorcerer in my party tries to use acid splash a lot but almost never hits.
Because the crossbow can hit targets a long way away it is worth getting. The range of acid splash is so short that most of the time that an enemy is in range he is also in melee with someone in your party.
If you want a cantrip to attack with I suggest daze. Though it only works on humanoids, it is a pretty effective save or suck at low levels, assuming your save DC is decent. And you don't need to roll to hit.
BTW a masterwork backpack is a waste for a low STR character trying to keep to a light load. It increases your effective STR by 1 but it weighs 4 pounds instead of 2, which eliminates most of the advantage for using it. You want to give the masterwork backpack to the half-orc fighter with 18 STR. It does way more for him. Then get him to carry your stuff.