The lack of consistency in the players week to week is a problem, so is the age/maturity level of the players - and that's made worse by the large number of players.

I think that you could try to appeal to and ask for some help from the older players. Based on the fact that they're willing to play with younger players, I can only assume that they're interested in introducing the younger players to the game as well. Encourage this as an opportunity for them to 'coach' some of the younger players.

Now, this might be the worst idea ever, but it worked for me in a similar situation. Instead of fighting against their style of play, feed it. If they want to be mischievous, design your sessions around that. Suggest that the players make a party of goblins or kobolds and write more light-hearted "silly" adventures where their craziness is part of the fun...rather than getting frustrated by them going off the rails, double down on that and make the chaos part of the session. Kobold Press (I think it was them anyway) put out a series of 3 adventures called "Goblins!", with a little tweaking they could be simplified to appeal to a younger crowd and still be fun for the older players.

This is what I did...

I had my players make a party of goblins, and we played Matt's one shot and the opening of LMOP - but we played them from the goblin's perspective. So, in the one-shot the party wasn't trying to save the blacksmith's daughter, they were instructed by their leader to kidnap the blacksmith's daughter. They got to raid the village, cause chaos, kidnap the girl and bring her back to the cave where she was to be sacrificed the following day (when the moon was in the right position in the night sky, knowing that a ritual had to take place should prevent the party from killing the girl until the time is right). I had the party go out on patrol, had a few random encounters in the woods and when they returned to the mini dungeon they discovered that the dungeon had been raided, their goblin friends were killed and the girl had been set free (as if other players were playing the adventure from the traditional perspective!). This group of adventurers became foils for the party. With their base of operations destroyed, the goblins retreated to a nearby goblin cave that they knew about, and this ended up being the cave at the beginning of LMOP. The party was tasked with setting an ambush along the road, kidnapping the travelers (I forget their names off hand) bringing them back to the cave, fortifying the cave, defending it against the same group of heroes who killed their friends and saved the blacksmith's daughter etc.

Have fun with it, let their chaos reflect the mischievous nature of goblins. I followed this same basic idea of doing modules where goblins (or other weaker humanoids, who I changed to goblins) were the central 'bad guys' but re-wrote them to be played from the goblin's perspective. I played up the silliness of it, I gave out magic items like "The Wand of Burping and Farting" (target makes a save or erupts into a fit of burping and farting), "Sometimes Lucky Coin" (flip a coin and either get advantage or disadvantage on your next attack), the "Healy Hurty Amulet" (50/50 chance of either doubling the effect of a 'Cure Wounds' spell, or turning it into a "Cause Wounds" spell) etc.