I use my hand through the clothing. Not super hard but it stings and they get the message.
+1
The belt is excessive. I believe that a good swat will suffice. Pain isn't required. When they're toddlers or younger the strike's
noise is effective. The ratio of loving touches to swats has to be a minimum of 100:1, otherwise they'll fear your hand.
I've never trained a dog with pain, and I certainly wouldn't subject a child to it.