Tell them to suck it up, life isn't easy and now it's time they learn. I'd make them walk around the neighborhood wearing a backpack with at least 25 lbs in it for a month or two prior to the trip. That walk should be at least a mile to start then work their way up to 3miles.
*****I'M TOTALLY KIDDING******
Our son was just about to turn 5 the first time we had no stroller. He did pretty well, but this was way pre-covid and crowds were low since it was the middle of August. I carried a back pack with tons of water and cooling rags, etc. lines were generally pretty short.
We also made sure we stopped and did rests and of course riding a lot of rides helped because he was off his feet. We'd go out from about 8am to noon, eat lunch then head back to the room for break/nap time. We'd go back out around 5pm , get dinner then hit up the parks.
The funniest thing was day 3 we had a dinner reservation at Jiko. He absolutely passed out and he ended up sleeping on the bench at the restaurant. My daughters loved it because they didn't have to deal with little brother. The worst part was carrying him to our room which I think was the farthest possibly away as we were in a 1 BR at Kidani Villas.
overall, he was really good and it wasn't a huge deal. There were a few times he wanted a stroller but we would just tell him we didn't have one and we'd go sit and relax for a bit.