So, Danny is adopted, and his birth parents are Severus Snape and Sirius Black. Sirius is the only one that knows of his existence but had left him with the Fenton's for his safety during the war. The Fenton's, unaware of the magical world, simply thought that Danny was put into their care because Sirius didn't trust his family not to harm him. When Sirius first breaks out, he sends Jack and Maddie a letter through muggle post, explaining everything. He says that he has some business to finish up in England first, but that he wishes to see and get to know his son. When Jack and Maddie receive the letter, they sit Danny down, and tell him he's adopted. I would imagine this happening shortly after Danny gets his powers. By the time Sirius is actually able to come and see him everything has happened (No Phantom Planet). While Harry deals with the Triwizard tournament debacle, Sirius is getting to know Danny. When he finds out Voldemort is back, he tells the Fenton's about the magical world, and the war. He also tells them about Danny's other dad and explains that it's normal in the magical world (mpreg). He then requests to take Danny with him, so that he may have an official magical school record. The Fenton's agree. In order for things to work timeline wise, Danny's a year younger than cannon when he gets his powers, or a year older than Harry. When they get to England, Sirius talks to Dumbledore, and gets Danny enrolled in fifth year, as Daniel James Black. That's where it really begins. You could also, as a separate piece, have more ghostly 'adventures' from while Sirius was there.