Like 1st grade at the lastest.
Typing is super easy..
Whatever you do, don't use a site or a game, especially "Type to Learn"
They are awful. And don't make your kids type using the homerow technique, I've been typing randomly and I am one of the fastest typers I know. Just have them type. It helps to have them type from their minds, not reading then typing, because it is slower and they will learn faster the other way.