I would not fret over this...are you kidding? Do you know how many millions of kids go to Disney?LOL. I would discretely (so as not to embarass your niece) tell the front desk at check-in, they will notify housekeeping. Housekeeping will probably place a plastic covering over the matress and no problem. If an "accident" occurs, simply remove the sheets so housekeeping knows. Once you know who your housekeeper is, I would pull them aside and thank them for any "extra effort" they may make. Don't forget to tip generously. Relax....during one trip,my daughter at age 5, vomited all over the room!!! Our housekeeper was very understanding and was compensated for this "extra" duty. This is one of the reasons why it is essential to be nice to your housekeeping staff. Explain to your niece that things like this happen...even Mickey Mouse wets the bed occassionally....shoot, I almost wet my pants on the TOT!!!!