Molly is cute, but Molly Mae is rather too far on the cutesy side. Still, she'll just be Molly Lastname to most people, and there's nothing wrong with that.
If I were them, I would have thought hard about moving away from M names. Do you have any idea if they purposefully limited themselves to M names or if it happened by accident--like Madelyn and Molly just happen to be the names they love best?