I would always tell anyone to avoid buying a Shetland Pony. 95% of them are total arseholes and completely unsuitable for children. There are two at my college, and one of them is a total nightmare. He'll relentlessly bite anyone trying to lead him unless they whack him one outright. I've seen a few Shetland Ponies who are fine with adults who can put them in line, but walk all over children when the adults aren't there to supervise.

The odd 5% that are sweet and well behaved are worth their weight in gold, and would only be for sale over their owner's dead body.