24 inch can mean several sizes, do you mean 507mm? I think the Plus version gives you much more flat protection than is needed on a childs bike. With the amount of weight a child puts on the tire, flats are less likely.
I have some older Schwalbe Marathons in 507 X 40 on my folding bike, but they have not made the 40mm width for several years now.
I think you would be looking at 47mm width.
https://www.schwalbetires.com/bike_t...s/marathon_420
Oops, I typed the above and posted it before I saw the posting that said that the bike can't take a tire that wide.