What Kids have and don't have at the same time.

Could someone please solve this riddle for me? My friend gave it to me, but I have forwarded it to some smart people at work who I thought would have a bettre shopt at it than me. I couldn't find any reference to it on the web on any riddle web site:

"Name something kids don't have but every kid has one."

Three clues. Each clue is a word:

Letter of alphabet
Type of person
Music group

Thanks!!!