I would rent a double for the parks for a 3 and 5 yr old. Just be prepared to carry them out of the park after returning the stroller, lol!
We rented a double for our 3yr old and 5 yr old last summer. I never would have done it but we had our infant with us and needed her full size stroller. My older two didn't fit in my double and personally, it was too heavy for me to push. Well, it ended up being the best decision I could have made. I loved the rental for my older ones!
I would definitely rent again. Their strollers are soooo easy to push and my kids loved being able to hop in and out. We used the net pockets for drinks without any problems as long as the drinks weren't really full.
I wouldn't rent for probably under 3 years. But for 3 yr olds and up, I really liked the rentals.
Oh, and we brought my maclaren umbrella stroller for my two older ones to share around the resort and outside the parks. I was glad to have that because they were exhausted from doing the parks and needed the stroller more than I thought they would outside the parks.