Olivia I definitely think that you should read Little Women first. Then you can read the MDBC and understand all the references and not have any surprises spoiled if you haven't reached that part in Little Women yet.
Brooke N. I see it being fun either way. You could read it ahead of time and know what the girls are talking about and speed through the book, or you could read it with them, however it would take longer to get through the book that way. Hope you enjoy it as much as I did!