Just like everyone else said, with little people plan for more potty breaks, plan for more time to stop and play, plan for more drink/ice cream/snack breaks, plan for more character interactions. Keep them hydrated and cool. They won't know to take random sips of water all day. Nothing's worse than an overheated kid puking everywhere. I always had the kids carry their own tiny backpacks so they could carry their own refillable water bottles, hand wipes and extra socks.
Also, the umbrella stroller thing is real. My kids are 13 and 15 now, but I brought the dirtiest, jankiest umbrella stroller with us until the youngest was 7. We'd only use it in the morning and late at night when they were tired and we had to walk long distances, otherwise, we'd just park it in labeled stroller parking. No one would touch that filthy thing.
Another thing, make sure they have really good footwear and extra socks. Cushioned runners, not Keds, not Converse, not flip flops. I feel so bad for the little people with aching feet. They don't know any better.