Chris 249 said..
The Windrush was an Australian board so won't be available there.
If there's open water and a significant chop you may be better off with a wider beginners board like a Starboard Go, or a Bic Techno 293.
They both have centreboards, which can be critical in keeping boards off the beach when they are using small sails.
I can recall the Sodim boards. The newer widestyle boards will be more stable in the chop, which is one area where the old narrow boards cause problems to beginners. In flat water the longboards can be great, and a Tiga or TC could be available at very low cost.
In your situation I'd be tempted to use the windsup at first, if it can be fitted with a simple fin, just to get a feel for the sport and for what your kids want. The 2.5 is good in some ways because a 13 year old can handle it easily, but they won't really get the feel for the power and for leaning back. A NP 3.7 wavesail is going to be heavier and underpowered for its size compared to a SUP sail like a Starboard SUP sail 4.5. These sails can be about half the weight of a "normal" sail because they don't have all those heavy battens, and with the deep shape they provide more power for their size (at the cost of being twitchier and slower in strong winds; everything is a compromise).
We've taught dozens of people and found that it can be very good to switch back and forth from a light, easily-handled sail to a more powerful one. They each teach different skills.
I agree with Chris,
he has plenty of instructing experience with kids. I also instruct kids and adults.
for a school, a wide board like a starboard start is essential for first lesson or two - especially with heavier or less co-ordinated students. Wider boards work better with the really small beginner rigs than a long board. The problem I've found with shorter wider boards in light winds is that even with a daggerboard, students tend to drift sideways a lot and don't get the forward drive until they advance a little. Great for establishing the basics however.
starboard Rios are good also and not a board board to buy as they are a good alrounder that the kids can develop on over a wide wind range.
ive also been teaching beginners on a Windsurfer LT with 3.5 or 4.5 beginner rigs, they are super light as they have tiny masts and booms. Sup style rigs, some older sails without battens but with short booms (not surf style more recreational low end sails) or things like the bic nova kits are good. The longer narrower board almost immediately gives them forward thrust once the sail gets a breeze. I've had first timers get on the LT (after demos and practice on simulator onshore) hop on, pull up a small rig sail away, turn and return to shore!
Best to find a cheap compromise board, not too narrow and for a 55kg kid a 4m beginner sail. I've found too small a sail on longer boards will hold back development for reasons mentioned by a few above. They need to feel the power and learn back and a long board needs a sail with a bit of power and full profile.
great your kids are interested. Enjoy the journey. They will be sailing some of your regular boards in no time.