Thank you all for the interesting replies.
I have seen "kid brother" used. For example:
Junglecity.com -
Jennifer: Who is he?
Erin: That's Michael's kid brother.
Jennifer: I didn't know he had a kid brother.
Erin: We hadn't seen him for years because he lives in the east coast. They aren't that close.
Is the dialog above unnatural to native speakers?
Don't anyone use it referring to "younger brother"?
Hirashin