As long as your kids' posts aren't locked or encrypted, you can count on Light to have listened to and watched them. But, again, that doesn't mean he'll be outright broadcasting the information he's collected on people - all the information he gathers about others will be solely for his own knowledge only.
As for Ribbons - I'm not entirely sure how encryption works with a 3-button device, apart from the obvious keying of a specific order of buttons and voice commands - but you can count on Light's interest in it, and he'll certainly try to hack into it.
Actually, as far as hacking is concerned, I haven't really poked any of the mods about that yet, so I'm not even sure if you can hack private posts here... But Light will definitely be keeping a close eye on any public posts!