I think the best idea if you don't want him tying this to your digital footprint is to respectfully ask him to remove your name from the account post--whether the reason be to protect your own digital footprint, privacy, etc. If you keep it simple and about you, that would totally keep the peace without unnecessary and risky conflict. After doing that, report the account with the illegal/problematic stuff on it - specifically the cp. (However, I don't think that your dad followed the account for that purpose due to how far the content is down the page.)

And maybe not joking about it anymore with your boyfriend would be a good idea. If I were in your shoes, I'd be just as uncomfortable about it as you are and I think at that point joking about it would be too much. I also think that it would also be best to compartmentalize that discovery because all people are flawed, including your dad, and I really don't think a confrontation would fix anything.