The only time there has been a problem is when people are checking in with free dining. For free dining reservations, everyone had to be present to make sure people weren't inventing children to get extra dining credits (since it didn't cost extra for the room to add children) and then giving those credits to adults (a loophole Disney has closed). Otherwise, it's no problem. It's easier if you are listed as the primary guest on the reservation, with your mom as the secondary guest.