How can I make sure my kid isn't sharing too much on Facebook or Instagram?
Take a two-pronged approach. First, probe a bit to find out if your kids might be at risk for oversharing. Reserve judgment until you've heard your kids out; a heavy-handed approach can lead to them shutting you out. Ask about what types of things they and their friends share. Make sure they're not feeling pressured to post things they're uncomfortable with. And discuss the risks of oversharing, which include damage to one's reputation and regrets about sharing personal information.
Second, check in about privacy settings. Kids don't always think through the consequences of their actions. That's when privacy settings really matter. Even if kids do think before they post, if their privacy settings aren't enabled (or aren't strict) they may be sharing more than they mean to.