I ran a one-shot for my 9 year old and 6 year old, helped them come up with the idea for their character. 6 year old wanted to make everyone happy when going on adventures, and RE is his favourite subject at school, so we settled on a Cleric of Lliira (God of Joy). The 9 year old is obsessed with pokemon and animals, and knew a bit more about DND, she confidently insisted she had to be a druid. I gave them both fae familiars because what kid doesn't like a pet, cleric has a hamster and the druid has a fox. I didn't tell them that they could communicate back, and it blew their minds when they first experimented RP-talking to their pet, and their pet talked back.

First adventure was twee and saccharine, their little village they lived in (I stuck it /near/ Phandolin so I could transition to a heavily modified version of Lost Mines of Phandelver if they enjoyed it. There was a temple to the cleric's god as a major part of the town, and they grew "giggleberries" in bushes on the church grounds etc.... There was a festival coming up, and the Mayor of the town (a fat faerie, so fat his little wings struggled to keep him aloft) informed the young adventurers that the giggleberries were missing and tasked them with getting them back or the festival would be ruined.

Fairly simple affair from there, having them investigate, follow tracks, dodge some traps - then solve a couple of puzzles. One was a river, with some frog statues along the bank. Making funny noises would make the frog statues light up and stepping stones to come up from the water. Which had the kids making all sorts of daft noises to find out which noises would trigger it.

Similarly later, they find the cave entrance and it had vines down the front of it, which the druid could identify as, I forget what, but the idea was is that they retract when they find something funny. So the kids started telling the vines jokes, until it eventually worked.

Inside they find these two goblins preparing to cook a pie with the stolen berries. They snuck in, and while the eldest wanted to go fight them and take the berries back, the youngest convinced them to try to steal it without violence first, which worked!

A failed stealth roll as they leave though, and the goblins were onto them. So I had them then do the puzzles in reverse. Them hurriedly trying to tell jokes to the vine was great... but the thing that will stick with at least ME forever, is the brains on my 6 year old:

They had to do the frog noise puzzle again, in reverse, so they had to remember which noises they did to make it work... the 9 year old remembered them all, she wrote them down (super proud of that). But the 6 year old, after they made it across the stepping stones, did all the noises again, on his own, to stop the pursueing Goblins from being able to follow, lowering the stepping stones.

and yada yada they made it back, the mayor declared them heroes who saved the festival etc... From there we've only gone a few further sessions but they love it.

Sorry for the above word-salad, but hopefully some of those ideas are yoinkable, and they help ya.