Thank u very much! =)

Unfortunately no such terms for thm. ;/

It's technically your "cousin once removed," but no one ever says that. Usually you would just say "this is my cousin's son," or "this is my cousin's daughter." Actually, we rarely even say that. We just call them by their first name and avoid talking about the relationship because it's so confusing that no one really knows what it actually is.

Your relative. Or, your cousin's daughter/son. It doesn't get really complex in English.