My son Luca developed perfect pitch, and I started at 2 with him. Simply by playing simple tunes on piano, and asking him to sing them back, in solfeggio. His range was do re mi back then, but he remembered them on spot, and he was able to sing me a straight E or C, even when we were not studying at all... I guess kids at that age have the ability to remember pitch as easy as any other thing.
Later, with Soft Mozart games, he learned even more. Now he is 6, and he can pretty much recognize all the notes, even when we take long breaks (like 1 month not touching the piano.
Good luck!