Begin Again Book 1.

Hecate, sensing danger approaching with the Wizarding World, decides to send the three of the most powerful demigods to grow up with Harry Potter-but when he is a child.

Percy Jackson, Jason Grace, and Nico di Angelo must grow up with Harry Potter to ensure the survival of the Wizarding race. Of course, to do that, Hecate would need to send them back in time, erase their memories, and turn back their time as well, so they are young.

It doesn't help that they, along with the rest of the Seven are minor gods now. Or that Percy and Jason have wives and families. Or that they are twenty one now.

All that matters is that the plan works. And it will. So long as all goes according to plan...