This is a loophole in Da Rules that lets Timmy Turner keep Cosmo, Wanda, and Poof. Jorgen and the Fairy Council presented it at the end of the Fairly Odd Movie. It states that if there is ever a boy named Timmy Turner, and if he has kept and loved his fairies for more than thirteen years (and has saved Fairy World a couple of times), and promises to make wishes only for others and not for his own selfish purposes, he may keep his fairies. With Timmy Turner becoming a fairy himself as of the end of "A Fairly Odd Summer", the loophole is presumably now null and void.
Since Timmy is now with Tootie, she now knows about Cosmo, Wanda, and Poof's existence. She is the only other non-fairy person who is allowed to know about fairies' existence, although Chester and A.J., who were also told of the fairies existence in the extended cut of the movie, may also still know of them as their memories are never indicated to have been wiped. Together, Timmy and his growing fairy family open up a business called "Wishful Thinking" and travel the world using fairy magic to make it a better place.