If this was a family member like a niece then maybe I could understand you guys hanging out sometimes...but this is a stranger that you met on an app right? 12 years difference wouldn't be as big of a deal if you were 24 and the other person was 36...but at 12 years old they are in such a different place in life than you are. What would you do together? Would you have anything in common? What would you talk about? I think it would be best to find adult friends. I wish you the best of luck.

You also have to consider how the child's parents would feel about you. Be careful.