Not as a daily thing... I think it'd ruin their young hair and start teaching them to be paranoid about looks before they should even start caring about these things... but as an occasional thing, I don't mind.
I had gay housemates for a while, and they would bribe DD with lollies so she'd let them straighten her hair. She was 3 at the time.
I've done it once or twice too when she's asked after seeing me do mine. When I was little I was always begging my mother to curl my hair any way she could - even with rags. I see it as no different to that really...
(And I also got my hair crimped on occasion, and felt amazing... despite probably looking like a bit of a twit... lol).