That is a hard one, you might look into ope source drive solutions, but that would require some setup and if you use a lot of space then also money.
To answer your question, I don't endorse the following solution, but I think it would still be 1% better than using Discord: If you already have Telegram, you can use Whitehole.
Q: Can I donate?
A: We appreciate that people want to support us, but we never have and never will accept donations. We maintain this project because its fun and we want to help others, not make money.
Via their FAQs.
I really hope you keep believing in the value of your app and never stop developing it. It might sound like a small step towards overall more privacy, but every kid who use your opensource and private app instead of the ads and trackers filled closed source apps is a person who is not benefiting the Capitalism which is killing people privacy.
So... Who to blame for this? The parents who did not care enough to restrict their kids phone time or the good developer who is trying to preserve kids privacy by offering a open-source game?
I just wanted to add my valuable insight to the discussion here: Fuck Cars.