I think it was Mangle, but a total accident. It probably started with a game of truth or dare. Since Mangle is the animatronic that the kids could touch the most, someone probably dared the victim to put their head in it's mouth. Then, it probably malfunctioned and bit the kid. This also explains the broken jaw.
And he or she might did it bc the children magling her or he all up
Even we animatronics don't know who caused it!!!
So our main suspects are @Foxy_The_Pirate_Fox and @Mangle_The_Fox