Is yours some very old model? If its a new model radiator, they dont get hot enough to be able to burn quickly.
Put another way, you would have to keep your hands firmly pressed for more than 5 seconds before anything would burn. Most little ones will touch it and in 1/2 second take their hands away; there may be a tear or 2 but no damage would be done.
For most kids, once they do this, they wont do it again.