It will take some "adjusting" for your kids to get really stable on the Starsurer S, but since they are kids. they should adapt very quickly.
At < 90 lbs, I'm sure they can progress, if you get them a good powerful rig that they can plane with.
The rig has to be very light in weight or they won't like uphauling it on the narrower and more "tippy" Starsurfer S.
You wife should be right at home on this board after sailing the Astro Rock.
Hope this helps,