If your parents' power chairs are their own, you should find out how old the batteries are. For scooters, the batteries last (reliably) about one year or less. Even tho they'll run after that time, they'll wear out quickly during use and need to be recharged frequently. Loss of battery power on a personal unit because the batteries are old, is a self-inflicted problem. It would be on you to track down where to get new batteries in the Orlando area and you'll lose park time. That's the only fix.
Best answer for you is to make sure your batteries are fairly fresh before taking the units to WDW. A trip to WDW will require battery power like no typical day ever calls for back home. If the batteries are fresh and the power chair is operating properly, the batteries will last all day--every day, with a full charging overnight--every night.