RE: Trying to choose a board for my kids...
I think the Rio Small will work quite well.
Yes, you children are small, but I've used the Rio S, on a tether behind
me on a Rio M or Start L and this works really well for any kid large enough to handle a rig and sail on their own.
As soon as they "prove themselves" by sailing along with lots of slack in the tether line, I untie the tether and they are on their own.
I would however recommend that you use a 1.7 and 2.5 m2 rigs for them, and limit their sailing to the < 12 knots range (until they prove they can handle more wind).
Kids are amazing, they can do almost anything.
They pickup new techinque and learn from what they see others doing practically by osmosis or something.
BUT, you need to stay on top of changing conditions and have you or your wife sail near them whenever they are "challenging" their personal boundaries, because if they get stuck out there, and get scared, it's often hard to get them back into it.
They are "fearless" up to a point, and as the parent or relavant adult on the scene you need to be close to support them when situations the simply cannot handle alone confront them.
Get them the best, easiest to use gear, send them out in conditions they have the skills to handle (they will amaze you with how quickly they can handle more daunting and heavy duty things) and they will learn sso fast it makes your head spin.
Send them out on gear that isn't easy to use, in conditions that cause them to become afraid, and they may simply quit and not want to try it again.
The right board and rig, good basic skills lessons, some time on a tether, whenever the conditions are daunting or new to them, and they will exceed your expectations at every turn.
Hope this helps,