Hi KimiCHHH,
I’m sorry to hear about the issue!
First thing first, our app is not a game, and it’s free to play, so if your child accidentally added it to your account, don’t worry, there was no payment involved. As I don’t know anything about your Steam account, I can’t verify whether you own any of our titles.
If there was a DLC purchase, then, unfortunately, there isn’t anything we can do, as Steam is in total control over all transactions, and they aren’t exposed to us, developers. Subsequently, we can’t offer a refund for them.
As far as I know, Steam has a no-refund policy on DLC purchases, but you can always raise a support ticket with them and ask. However, they generally don’t buy the “someone else used my credit card” story, so a better explanation of why you don’t like your purchase will be needed, even if it is the truth.
I hope this helps, even though this isn’t the answer you were hoping to get. Please let me know if there’s anything else I can help you with!