It seems as though all the baby characters are included, as well as deeper cut characters like Birdo, King Boo, Lakitu, and Shy Guy. More notably, it appears as though Dry Bones wasn't included in ...

more bashful Boo, along with Dry Bones and a yellow baby Yoshi. As with other expansion sets, you’ll need one of the original Mario, Luigi, or Peach starter set figures to fully experience this one.