> log into their accounts, having access to their usernames so that you can do it, so that you know what they're up to.

Wouldn't it damage the trust between you and your child? I think even monitoring internet usage in-person or outright restricting access to potentially dangerous websites would be less intrusive. If your kid discovers you can access their messages at any time, it would make them more inclined to make other accounts and not discuss about any dangerous interactions on them out of fear of being discovered.