To stroller. Yes or no?
I am going down with 15, 13, 11, and 8 year old children. I have a City Elite that I was going to use for the 8 year old and to hold gear. But now other than for Animal Kingdom I am planning a midday break, about 3-4 hours, leaving the park and returning in the early evening (until 11 or midnight.) All of the kids have cinch bags so should I just have them carry their own water bottles, snacks, ponchos, whatever and forego the stroller?
I would go without the stroller - with your planned breaks an 8yo should be able to cope.
But, to be fair, I am in the anti-stroller camp and feel that all strollers other than the umbrella-style folders should be banned from the parks. No more Maclaren monstrosities the size of RVs clogging the walkways and running into your ankles! 
We went for the first time when our youngest was 6 (he’s now 8), and did long days, usually without a break. No stroller. We carry one backpack with necessities & trade off. The youngest DOES beg a piggyback from the oldest when he gets really tired. 
No Dd7 did fine first visit no stroller Only got carried once and that was after wishes on day 12 after doing rd most days.
Be free! No stroller.
We had a Maclaren Volo stroller with DS5, and were glad to have it. When we went in Jan, it was nice to have a place to store the jackets, water bottles, and our snacks. When we went in June, we kept ponchos and crocs (our pouring-rain gear) in the stroller.
I’ll also put in a vote for no stroller.
We had planned to use a double stroller for our almost six-year old twins. After eye-balling MK from the outside the night before, we decided to give it a go without a stroller. Also, we were comfortable in the knowledge that if it didn’t work out, we could always rent one there.
I am happy to report we never needed a stroller the entire trip: 2 1/2 days at MK, 1 full day at DAK, and 1/2 day at Epcot (though we did not go around the World Showcase). And one backpack was all we needed for all our stuff (I realize you have more people to worry about). We only carried our kids once and that was on our shoulders so they could see the electrical parade one night as we left MK. Otherwise, they would’ve been fine to walk.
We just did a trip in July with DDs 8. No stroller. They each carried a string backpack with water, frog tog & coins for pressed pennies. No complaints, but we’re did take a midday break each day.
Is your 8 year old of typical abilities? Does he/she use a stroller for anything at home?
For a typical 8 year old I wouldn’t bother with a stroller. As long as you are taking appropriate breaks and not pushing too hard during the day I wouldn’t be concerned that the 8 yo can’t keep up.