So your argument is that you can reach in time to slap, but not in time to yank away ?  Now I have a three year old, they aren't *that* fast - seriously I can't imagine a scenario where it would be possible to do the one and not the other.
What I can tell you is that, should such a scenario happen, you would not be guilty of any crime anyway - even now with the law changed, the good samaritan exception means that physical contact to prevent imminent harm or death is neither a civil nor criminal offense.