That "tripped and accidentally kicked his leg" thing is making me think she might have been tormenting the dog.
Does it change anything materially if the little girl pulled his tail and poked him with a stick?
A strange dog may react in strange ways. Unknown ways. Risky and dangerous ways. A dog IS an ANIMAL. And that this dog was chained up, relates that it was not trained to know its own limits. A dog trained, does not need a chain. A dog chained, has not been trained. And leaving your children with an unknown, untrained adult dog, without regard to the breed, is dangerous. The breed makes is more so.
A trained dog, raised from a pup with the children, could easily put up with a pulled tail, ears, falls, kicks, and all the behaviors natural to children who play with dogs. An untrained dog is, in and of itself, a dangerous animal. A trained on, only “less” so.