Yes, absolutely! If you want to bring your kids, feel free! They're invited as long as you are!
You may bring a plus one, as long as you let us know that you are! We need to make enough food for everyone, after all.
To a point, yes. We don't want everyone being so formal that they're uncomfortable — we want everyone to have fun and celebrate! As long as it looks nice and you're comfortable, you'll be just fine!
Ideally, you should RSVP by the week before the wedding at the latest. We understand if last minute things arise, though!