Why, you pile them in one of these, of course.
Our school doesn’t use them because we don’t take kids to the park until they can walk properly on their own, but I seem to recall seeing them pretty full with about eight kids in there at a time.
If you look closely, you can see there are separate bicycle hand brakes for each of the large wheels on the sides, and the four small wheels are just swiveling casters.
It also looks like the center piece opposite the handles opens up to let kids in and out, but I’ve only seen kids lifted in and out, so I don’t know for sure.
I took these two pictures last week at the park near MSP, when another local preschool brought a large group of kids at the same time our bilingual classes were there.