Not sure if I need to hide spoiler in the forum, but ...

First time DMing tonight with my 8 year old son. We ended tonight after he finished off the larger spider with a crit hit. I am playing Valeros and he is playing Kyra. If his aunt comes over tomorrow I am going to say we found her (Merisiel) in one of the webs. That is the main reason we decided to stop there. I had to lead him a lot which is expected but we had a lot of fun.

Your idea sounds great. When I GMed the adventure, I didn't give the treasure to my players because the went the diplomatic route. However, I gave them bonus XP (on top of the encounter XP). I did this in part because I like to encourage solutions that don't go the traditional "Kill'em all!" path but also because it was to offset the loss of treasure.

I had them set an ambush for me when i came back from black fangs lair. We ran and we not able to defeat him the first go. when we returned we had there treasure to help us and it worked.

Trading treasure for the toy, bonus XP for being diplomatic, and having the goblins ambush the PCs later all are good suggestions, in my opinion. :)

The above is what my group did, although we successfully intimidated the goblins on the way back.

Maybe the Goblins are overjoyed from the toy and are too distracted to see some adventurers steal their treasure, also goblins are idiots so they'd have no idea about what happened to it a few minutes later

Here's how my players' encounter with the goblins went:

The rogue stealthed around the entrance to see them (after hearing the voices). She then wanted to throw one of her daggers at one, hoping to have them funnel through the doorway to the fountain room and be picked off by the party one-by-one.

She missed. Rather than alarming the goblins, I said the dagger landed in a pile of soft dirt making almost no sound. Since the goblins were distracted (arguing over the lost toy, digging through their treasure chest), they didn't notice. Rather than throw another dagger, the party decided to all enter the room.

As they noticed the goblins fumbling around in the treasure chest, Fatmouth sees them and shouts, "Who you?!"

The players were pretty confused that the goblins were staying put and talking to them instead of rushing for battle upon sight. The conversation went well, so the players went to find the toy.

When they brought the toy back, Fatmouth was pleased, but still acted like an arrogant dick towards them. I mentioned that Fatmouth looked back at the treasure chest, then back at the players with a suspicious look. He tells them he won't kill them since it will benefit him if they kill the dragon. He commands them to be on their way.

While attempting to climb the cliff, a couple players fell and took a bit of damage. I made sure to let the players know how much the goblins were laughing at their misfortune. I was trying hard to get the players to start a fight since I wanted them to have the treasure for the fight with Black Fang.

The party was in rough shape for hit points, and the cleric could only do so much to heal them. They had already used the potion of cure light wounds earlier in the dungeon. I wondered out loud if there might be anything in the goblin's chest to help them with that (but didn't specifically mention the wand of cure light wounds).

The rogue decided to drink the potion of invisibilty. She went back down the rope, snuck over to the chest, and while the goblins were fighting over the toy, she lifted the items from the chest and used the rope to easily climb back up the cliff to rejoin the party.

After the encounter with Black Fang, they had to make their way back through the goblin room to leave the dungeon. Upon reaching the bottom of the cliff, Fatmouth immediately accused them of somehow stealing his treasure. They tried to deny it, but the angry irrational king was in the mood for battle now even if they were telling the truth. The players killed the lot of them and got extra XP.

I've taken the beginner box set and added it into the story line for my home game. Most of my PCs have table top/RPG experience so I didn't have to start with the beginner characters or simplify stuff too much.

However my mistake and a lesson learned was letting them roll for Stats. Ya I have to toughen up the mobs in the modules as they can hit and blow through CR 1 mobs.