You'll want the stroller for the 7 yr old. Do not second think this. Bring a cheap 30$ umbrella stroller or a fancy city mini. But do not leave home without one. Repeat. Your 7 yr old needs a stroller. (Lol my dd8 was jealous of her dd1 sis sitting in it the whole time. Occasionally dd8 put dd1 on her lap n dad. Pushed.)
And this is where it's all very personal to the actual child. We brought our Volo, just in case my then 6.5 year old needed it. He had needed it during our last visit to DLR, so we wanted it just in case. It never got unfolded, let alone used. He was totally and completely fine, never once asked for it. And while he's an active kid, he's also prone to nasty leg-aches (as am I, as was my mom before me...no one knows why they happen to us) from walking a lot. But we used arnica lotion and all was fine.
OP I wouldn't bring a stroller just for stuff, because we stopped leaving stuff in the stroller. People take things! Squirrels crawl into the stroller and search for things. Birds land on them and search for things. We bring nothing we can't carry.
I have sunscreen in 3.4 oz bottles for the flight (I also carry big bottles in a checked bag but I want to make sure I have some from when we land) and I bring one or two of those with me, and various other things.
We bring our Igloo MaxCold backpack cooler, put frozen bottles of water in there along with cold VitaminWater bottles. YES there are ice waters available, but that requires waiting in line when you're probably already thirsty. We get that water with meals, but it's the in-between-meals thirst I want to take care of quickly.
So we carry stuff, but that's what we do, carry it. We take nothing that has to be left, because even when using the stroller we didn't leave things in the stroller, after dealing with theft twice at
Disneyland (but it happens in Orlando too).