Since your children are still growing, the Rio M might be the better call.
What size and type of rig are they using?
What problem were they having with staying upwind?
If they were constantly being swept downwind, where they taught to "tip" your
Futura 144 slighty upwind rail down so it will carve upwind on the shape in the bottom?
At 60 Kg. (132.2 lbs) the Rio S will surely work, but it will not be as stable due to the
narrower width (76 cm vs 80.5 for the Rio M).
I do not have the Rio S to compare, but I'm now on my second "new generation" Rio M
and I think the speed of the Rio M in shortboard mode (centerboard up and stowed) will
Bft F4 (11-15 knots) will favor the Rio M (particularly if your kids grow significantly) and
should have them planing with 6.5-7.5m2 rigs.
Hope this helps,