Take the kid to local events, check the pokemon website and go to event locator. They are usually well labeled so avoid a "challenge" or a "cup" but i can almost guarantee* if someone's opening packs at the store your kid will get some bulk for free.

*i give my bulk to kids and i see it happen, hopefully your area is similar

For casual play and deckbuilding the prerelase kits are usually for sale from older sets. Some are overpriced but if you find a reasonable couple boxes they contain one of a few different 40 card premade decks and 4 booster packs. For prerelease you get to swap any cards from the packs into the deck and play a tournament so they're meant to be playable and simple since they're intended to be the first look at a new set.

If you lived near me I'd just drop a box of cards on your head and thank you for taking them off my hands tbh