As long as you pop the kid's backside then it is perfectly legal in all 50 states.
Is it legal to pop a tantrum throwing 10 year old with a belt in the state of Florida if talking and reasoning isn't working?
I wouldn't recommend it. Not only does it make the child even more upset, but you could seriously hurt them. In my personal opinion, taking a belt to a child is like taking a boot to a kitten.
But, that's just how I feel. As for legality, I would assume yes.
