You can buy those transaxels (different number of speeds) surplus fairly cheap, but you have to look.

Though the goal was a kids toy, and those mowers are too large for that use.

This is one I know of off the top of my head: https://surpluscenter.com/power-transmission/transaxles-tran...

Kids grow. If it were me I'd just remove the mower deck, throw an extra muffler in its place (because without the deck the engine will be the next loudest thing and I don't wanna listen to it). Maybe lock out the top speeds depending upon age/yard topology.

Stuff goes straight to permanent memory at that age so by giving them a "real" tractor there's a lot of potential to learn good lifelong lessons prompt them to ask the kind of questions that result in good teaching.