You should ask the father for his name and his attorney’s name, as well as the name of their doctor and the medical records for the girl. Call his bluff. This is bullshit.
It smells like a scam. If you’re using a reputable dating site that requires all users to be 18+, then you’ve certainly got a reasonable argument to make, that you were acting in good faith. That said, I’m not a lawyer and don’t have all of the facts.
I’d probably call his bluff and tell him you want to go discuss the matter with the police. I’d say I’m going to have them contact you about your demands for money. That will get him to leave you alone.
Which dating site? Did you send her sexually explicit language or images by text? Did she tell you her age? Did you meet her in person?
There’s a ton you’re leaving out here. If you did something illegal, even without knowing her actual age, you could be in a lot of trouble. Lots of advice here to “call his bluff”, but be very careful if you’ve done something that could be used against you. Any potential for ending up on the sex offender registry??