"C'est mon dernier garçon."
Not necessarily, if you coach a sports team, the last boy can be the one who arrived after the others.
no, "my girl" is not "ma petite amie" and "my boy" is not "mon petit ami".
think of a sports coach talking about the last boy having joined the team.
in specific context, it could also be a mother pointing to her younger son.
Why is "It is my latest boy" not correct? A translation of 'dernier' is 'latest'