I think the fact that she didn’t actually see it happen is annoying me as well.
To be honest, this is the most concerning part..
An old dog, who is startled, trod on or grabbed hold of from behind (?), may swing round and snap instinctively, it doesn't mean he is an aggressive dog, but no one should be leaving their three year old alone with their dog, so this is a HUGE safeguarding problem seeing as she is a bloody childminder FGS.
I wouldn't be sending her back, but I don't think the dog should be destroyed either, but she should definitely be keeping the two things separate anyway, especially if she is not actively supervising, it's a recipe for disaster.
Our 4YO grabbed our dogs tail as he was passing last week and he instinctively swung round, because of where she was his quick swinging motion clouted her in the face, but there was no biting or aggression, it was just an impulsive reaction to someone hurting him, most dogs have that instinct.