Flat iron? Bad idea.
I just received a comment via my Kijiji ad from a mother who tried using a flat iron to get rid of her daughter's head lice. She was surprised to still find 8 live bugs on the head after the ironing. So many people I meet try this, and I can't believe I have never written about it before. (So, thanks for the comment, L.)
My message is short but sweet. DO NOT use flat irons or curling irons to try to get rid of head lice. Head lice live right near or on the scalp. Your iron won't get close enough to kill them and if you try, someone is going to get burned. If you want a tool that is safer and more effective, use a blow dryer. (Don't do this without checking out more info here.)