I'm no legal expert either, but negligence always seemed more common sense than legalistic to me. It's nearly impossible to define in statute.
Would a "reasonable" parent in this situation have acted as they did, or not? Not being a parent, I wouldn't like to make a call on that.
I do remember looking after a 5 year old niece in a restaurant once, who when I turned my back for a second to see to my nephew who was crying she somehow ended up at the top of a long flight of marble stairs about to take a header down them. I (just) caught her in time, but had she fallen would I have been responsible or negligent?