Can I ask you how old your kids are?
If they are younger than 12, then I don't think it's good to waste their time for mad typing skillz.
I've learned through Typequick, though the software is kinda obsolete and costly these days. UltraKey as an alternative, maybe?
Anyway I don't think you need that kind of "gaming" feature is needed in learning how to type, mostly because if you're improving then that will be motivation itself. Also it's not good to just aim at higher scores in game, not at a proper skill.