Do you think such a law would create a problem when parents are accused of smacking their children when trying to slap a child's/teenager's hand away from a hot stove?

I think there would have to be defences built into the law, but I rather take that as a given. We should study what has happened in countries which have introduced such laws and take lessons from those. There would doubtless be occasional silly court cases, but that happens with most laws.