We rented a stroller for DD6 when we were there a week ago. Although expensive, I was very glad we did. It was good for a couple reasons.
It was easier to keep track of her especially when crowds were thick.
It was a holder for our extra stuff that we carried. I carried a Baggalinni with our camera's and wallet in it and left snacks and and drinks and whatever other stuff we had with the stroller when we went on rides. Made a much lighter bag for me to carry on me.
She would have been pretty good about walking, but there was a couple of times that she took catnaps between stops and it was good for her. She hasnt knapped since 2yrs, but with all the activity at WDW it gave her some zone out time while we walked.
I dont know what to tell you about your 8yr old. I think this was the last time she will really fit. She is a tall 6 yr old 48+ inches, but is thin. Her head actually pushed up on the canopy of the stroller and her legs were rather scrunched.
Another thing to consider is maybe buy one at home that is a little larger, then when you get home sell it on craigs list. You will probably not get your whole amount back but you will probably not loose as much as what you pay daily at the parks. We didnt do this because we flew and didnt want to hassle with it at the airport and have to wiat for it on a conecting flight.
Overall, I would do it again in a heartbeat if she would still fit into it. We were on the go a lot. Its not that she couldnt do it, but we wanted her to enjoy her time without pushing her, and this gave her a chance to rest inbetween attractions.