I agree that buying MBs upon arriving is a good backup plan if you really feel you'll want them. Or, just pack old ones if you have some. They'll still work to do the basics like unlocking your room and entering parks, just may not pick up in-ride photos or videos if more than 2 years old.
You can also ask for the good ol' KTTW card (Key To The World) at your resort's front desk. It's still free and does most everything an MB does. Certainly would be an easy stopgap if you don't want to run about to find MBs you like immediately upon arriving. Those are also very easily replaced if lost, inexpensive for loss-prone kids.
I actually found I used the 50th KTTW we both got as souvenirs almost as much as my MB. In some instances, it was easier to yank the card out of a pocket than to twist my wrist as awkwardly as sometimes necessary.
