I think that would be fun! The twins look a slight bit older than Manon, but young kids in various ages are still able to befriend each other and get along.
I'm really grateful we got a glimpse of the twins in the Christmas special. Considering Alya's future path in the series, it's fantastic that her family is being revealed more, especially as she mentioned her younger siblings at the beginning of Season 1. If we get to see Alya babysitting them, or Marinette with/without her babysitting the twins and Manon, it'll be a blast.
Oh my goodness, that would be hilarious. If Alya can't handle the Cesiare twins then who can. Hint: Totally not Marinette or maybe Alya would do the move she's famous for and call Adrien to come and help Marinette.