RE: first board for kids
The Rio S could be a good choice as a first board, but as you suggest getting a first board that is too large, or has limited "post beginner"
performance, makes for a less than desireable situation.
Are there any windsurfing schools in your area of Spain?
Might be a better idea to take the kids to a good school and get them through the basic lessons (better with a profesional instructor who's not part of their family like mom/dad.
If your friend insists on teaching them, then if both kids are under 120 lbs. the Rio S could be a good choice.
If the 14 yr old is larger heavier, then the Rio M would be a better choice.
Be sure that they are taught on really lightweight modern trainer sails.
Lightweight powerful trainer sails make all the difference when teaching youngsters (oldsters as well).
The Rio's are a little "limited" in the advanced performance issues, so you might want to look at the Starsurfer M as it's a great little board to learn on, but has alot of potential for advancement.
It's really the Kombat Aero Hull with an EVA deck and center fin box.
Hope this helps,