An RPG.
You can keep the RPG basic like Paper Mario, keep it short too to keep the job small; however, for multiple kids to join in the design step, they can design worlds, characters, quests, jobs and so on. Kids want to make Skibidi Toilet a quest in order to fight it with.... IDK, Mr.Beast bot? Let'em, as you might discover they will more than likely make jobs and quest's based on whatever their minds get hopped up on.
Everyone can also take part in the project making characters, drawing up small towns and cities, weapons, oddities, choice in music and choosing sound effects or making them.... Alternatively, you might need to have grab a sound pack with music from Humble Bundle when they start selling them in another bundle again.