The way I see it, is that if you're doing it yourself, you can't tell your kids that they're not allowed to do it. That's why parents who smoke never have any success with getting their kids to stay away from it if their kids want to smoke.
So if I had a kid who wanted to stretch a piercing, I'd let them do it, but they'd have to have a certain age. As some people have mentioned already, doing things to your body, that may be permanent, isn't a good idea at a young age. But when they'd turned 16, I'd just say 'go for it!' and I'd of course make sure that they took proper care of the piercing and wore proper jewellery