If it were me, I wouldn't be pushing 100lbs of kids around all day. I know kids get tired, so I would just take a rest if needed. If I were going to rent a stroller, I would only get a single, and have them take turns. I have twins who are 4, and I have a good stroller, but if I push both of them around it's like 80lbs! It kills my back and wears me out, even in a good stroller.
I have 5 kids, girls 7 and 9, twin 4 year olds, and a baby. We went in May, and they walked

We do have a baby, so we took one stroller for her. I find it's harder to get around w/ a stroller, and especially when I had to use the twin stroller when my twins were little. You have to go and park it, go back and get it, ect. With them walking you will be quicker into lines and on to the next ride. You may find you like them being more mobile, not having to lug so much around. I would think it would be SO much easier to navigate thru the crowds w/ walkers vs a stroller.