I'm a staff owner and as you know their awesome with people,especially kids but other dogs/cats/birds a real no no.8 years old and trying to walk a boxer might have been too soon for the little girl,their strong tough dogs and more than likely felt they were protecting the little girl in some way.I've not had a dog long but their clever little fuckers,mines 8 years old and knows it all,who to goto when she wants food,who to goto when she wants a walk etc etc. When I walk her she keeps right by my side,tries to pull a bit but very well behaved,I call her,she stops or comes to me,but when the missus or the boy walk her she pulls them all the way and totally ignores anything they donas she knows she gets away with it with them or she gets a smacked bum with me!!mate of mine had the same type of dog and told me that a boxer needs to know your smell first otherwise they can get aggressive,especially if you try to touch their head,perhaps that was why?don't put it down as that's a bit extreme for an isolated incident but make sure the dog knows he's done wrong,perhaps keep him in for a day or leave him out back without walking,always works with my little bella!!!