Best small horse for children?

Well, Grandpa has started a ranch (with all the requisite animals and then some). Currently he has two thoroughbreds (former race horses), an old mare that used to do kid-duty at a now-defunct stable and a small, non-rideable donkey. As grandchildren are starting to enter the picture, he will want to add another horse to the stable (Misty being almost 20yo there's likely not a bunch of kiddie-rides in her future). So: if you were to buy a horse for children to learn to ride, what breed and would you choose and why?