J.M.
My DD gets heat rash very easily too. Hers is a little bumpy, but not itchy or blistered.
Does the rash linger, or does it go away as she cools off? I had a friend who, when she overheated, would get VERY red in the face and torso area. It looked like a rash, but it was really just how her body handled the heat.
If it IS heat rash... My DD's pediatrician gave me this advice...
~Dress her in looser/flowy clothing.
~Wipe her down with a cool rag when she seems to be overly warm.
~Make sure she is getting enough water.
She said that heat rash is caused when a child's body sweats to cool itself off, but their pores are too small to handle the amount of sweat. So the sweat can't come out right, and gets trapped (which is what forms the bumps.)
Good luck! :)