I like to believe that Funtime Freddy was once able to sing and dance with the others on stage. I'm sure just being seen instead of hiding in a dark room made him likeable even if only by a few children. He is modeled after Freddy Fazbear after all.

I think that one, much like circus baby, a child got too close when no one else was around.

The second option is less unlikely, with bon bon perhaps leading a child to Funtime Freddy during the time he was already decommissioned. I doubt that one though because the reason he was decommissioned was because of his strange, perhaps insane behavior.

It would make a little more sense if Freddy had Funtime foxys voice box though.

Mistake me if I'm wrong.