Simplify the following text for a Grade 9 native English-speaking person.
Original: More recently , he has appeared in Mario & Luigi : Partners in Time , in which Baby Mario appeared via time travel along with Baby Luigi , Baby Peach , and Baby Bowser , which could explain the separation of the two characters .
Simplified: Baby Mario has a major role along with Baby Luigi in Mario & Luigi : Partners in Time and appears in Yoshi 's Island DS .