Our newborn daughter has her own mileage plus number. You have to start them early. I can log into her account just fine and do all of the normal things that you can do there. I can even book tickets through her account for her and one of us to fly with her. The problem occurs when trying to add her number to reservations booked through my account or as a traveler on the account. When you go to put in her MP# and hit submit it kicks back "Please enter in a valid MileagePlus Number". I have called reservations on this and they cannot figure out why it won't take and MileagePlus also seems unable to add the number to the reservations or traveler information. Has anyone ever encountered this before and if so any thoughts on a way to fix the problem. If it matters the MP number is 3 letters and 5 numbers.