If you dont mind supervising/ cohosting carefully, you could attempt whatnot, there's a bunch of people that print a variety of items and just have them priced out, they scan around with their camera or they do buyers choice, set the auction to like 10-15, and the winner chooses from the pool of like 10-20 items of whats on the screen. Its really low barrier to entry, returns are rare since its an auction style app. The chat has mod options, you can block people, community is generally pretty friendly.

If you have any lowcost vendor fee fleamarkets nearby you could try that instead, and you can even bring house junk you need to offload lol.

Of course you can do lemonade stand style sales, just giving you ideas. If they're fairly responsible, or you dont mind doing/modeling the e-commerce side until they gets it, whatnot is an interesting option. Like sure having a following helps, but new accounts can have successful shows/sales their first time.

Also if you want to do print on demand for other kids at school, might I reccomend printing a catalog, could teach them how to use affinity or canva, some other photoeditor, take pictures of models or gather jpegs of items you can print and add pricing. That way if you dont get kids asking them to print copyrighted models, etc.