Far as I can tell the Millennium Kids  are the Millennium Girls and Millennium Boys
which were based on the old Victoria 2.0 mesh.
There are a preteen version and a preschool version

Where-as the Millennium Kids Young Teens are based on the unimesh figures
like Victoria 3.0 of which  Laura is the Millennium Teens girl and the Millennium Girls Pre Teen
and  luke is the Millennium Teens  boy and the Millennium Boys Pre Teen.
And there is a separate set called Maddie and Matt which are the preschool versions
of the unimesh figures.