Hi Kendra,
How exciting, less than two months until your family's Disney World vacation.
While I have not personally rented from
Orlando Stroller Rental, I know a number of my fellow Moms who have. They all seem to love the ease and convenience as well as the quality of the strollers. I have spent park time with friends using Orlando Strollers and I can tell you that the stroller seemed plenty big enough for their 6 and 8 year old. The company's description, on their website, of their double stroller, lists the maximum weight capacity at 100 lbs so I do not think that will be an issue for your children. My only concern would be with your childrens' heights.
Personally my 8 and 10 year old boys are still able to fit into the double strollers, similar to the one you would be renting, as well as the Disney double stroller, however they both have problems with the height of the canopy and being able to sit straight up. Now my boys are a little on the tall side so if you oldest is on the "small" side, or the canopy does collapse, then I do not think you will have a problem. If you have any concerns, I would contact Orlando Stroller directly at 1-800-281-0884.
Wishing you and your family a wonderful time!