Your first,second, or third cousin's child, or, as a literal description of a person's much younger cousin.
How'm I gonna hang out with my baby cousin? We're from completely different generations.
Your sister or brother's child, who exists as one of the 11 recognized genders, but you can't really be bothered to think too much about it, so yeah; baby cousin.
Yeah, you know Ronnie's friend AJ? Her man's...... Baby cousin