When my daughter was around 3 or so, she had a nail-biting period. That was also the age she started to ask for nail polish. I told ther that she'd only be allowed to wear it, if she would stop biting her nails.
It worked wonders.
My older son was totally puzzled at my colorful toenails. I never painted them in front of him. He sooo wanted his nails polished as well. I'm not into little boys wearing nail polish, and DH, well, he would frankly have a fit.