I don't understand the riding-on-a-donkey simplicity stuff. So far as I know, horses of the roman empire at that time did not have backs strong enough to take riders. That's why the military used chariots. If one was going to ride on an animal, a donkey was the only choice. It was the barbarians from the Russian plains who introduced cavalry.
The striking feature of the entry into Jerusalem is not that Jesus was humble. It was his awareness that this was the last act. I don't see a way (or a need) to avoid the seriousness of that. Children need to think - and enjoy it, as well as to have fun. Without getting macabre, one could divide the Sunday School into two or three groups by age. Teachers could tell them very briefly what the entry into Jerusalem meant for Jesus. Then let the children discuss what they would do if there was one thing they could do for the whole world (or their families, or their communities).
Let them briefly report their ideas from the pulpit - either through spokespeople or individually (depending on the size of the SS.)
They will love being up front. And the parents will get a big kick out of it. And you could get some very insightful answers.
Incidentally, it's possible that horses originated in North America, but became extinct here.