As long as you’ll be in good terms, you can do anything! I can’t give any specific inspiration, because YOUR idea will be the strongest for you. Don’t worry if there are no ideas coming to your head, it takes some time to have an inspiration for the project.
Based on the informations provided, you could consider creating an obby, a tycoon, or any small fun/AFK games, such as slap battle, tongue battle, blade ball, collect all pet, or an RNG. These types of games are well-suited for your current experience and target audience. I’m not sure how much time you can dedicate to your game, but these options don’t necessarily require a lot of work and frequent updates.
you can ask ChatGPT unless u dont want to. For that target audience I would say a simulator or a story game or something fun. I would say RPG but those are A LOT of work and 9-12 year olds are not mature enough to appreciate them
Well then, based on Experience, you can make anything. I’d say to make something fun. Maybe make a small horror game? Personally, I find making lore to be extremely exciting, so you could try that out.