Has anyone had success SUP downwind with a kite? Is it worth the bother? I figure a slow kite down, paddle back type thing. Bring the kids and air out the 17m.
Not really worth the bother to SUK. I have done it and it is pretty worthless. It is also difficult. You no longer have an "edge" to dig in instantaneously as voluminous rails have too much buoyancy. This makes any huge board harder to kite than a small board. Couple this with the reduced performance of a larger board, and there is just no reason for almost every kiter to do SUK.
The kids thing may be different. I have used a 3.5m water relaunchable trainer with a passenger. No planing, and no upwind, but interesting. If you do have success with a full sized kite and passenger, post here about it. But make sure you stay safe and even more importantly, you keep your passenger safe.
I would actually recommend a windsurfing sail for the kid. If they are old enough for you to consider taking them out while you have a kite, then they should be old enough to handle a tiny kids windsurfing sail. The benefit of it is that they can do something that is cool and different instead of that uncool stuff that dad does. You can even show them videos on windsurfing and get them excited about figuring out things for themselves. If you are an ex-windsurfer, don't help them too much or "force" them do do it. Let them figure out what they want to do.