the correct way to say this is : "What did you want to be when you were a little kid" The comma before "when" is not necessary or correct. Sometimes a comma can be used when there is a natural pause in the reading of the sentence, but not here. No comma is needed. Yes , the sentence is a little long, but it's OK. To use the conditional tense would require an "if" clause towards the end of the sentence. Example: What would you do if someone broke into your house with a knife?"

When I was a child, I wanted to be an astronaut. When I finally found out that they had to have perfect eyesight, I was devastated! I wear pretty strong eyeglasses.

A pro-wrestler!

I wanted to be a great Inventor. I'd invent something that would be helpful to everyone.

No, Boruc, I didn't want to be a princess, I wanted to be a king when I was three.;~) But my father said, only men could be kings, so later I wanted to be an astronomer but then my mother told me I couldn't be one because I couldn't see the stars (I'm very short-sighted indeed). I had no luck with my parents :D