How do you say cool kid in spanish?

I would use:

"Eres guay" (Air-es gw-eye)

Or "Estas guay"

If they are cool just at that particular moment that you told them that, use the first one. If they are just a cool person all the time, use the second.

Other Words that can mean "cool" (an admirable state) include genial, bueno, mola (Spanish slang "you rock"), or the colloquial Mexican term "chido" (as in ¡Qué chido! meaning How cool!).