jedimistress, why would you let children corner your dog,
and why would your sister let her children climb all over their dog?
It always amazes me when people say their dogs will let their children do anything to them, why should a dog have to be subjected to having things done to them by unruly children - in these cases I would not blame a dog for growling at a child - although I would not expect it to snap or bite, just give a warning growl which is what they would do to another animal in the same circumstances.
My dogs are not very used to children, but provided the children treat them with respect, I would not hesitate to allow them to approach my dogs and touch them, under supervision.