I'm really only talking about SUP racing rather than kids getting into surf sup. I'd prefer my kids to ride short boards in the surf.
I'm not sure that SUP is uncool amongst kids, at least for pre teens anyway. Both my kids aged 8 & 11 have been invited to SUP birthday parties on Currumbin creek and at our recent training group Xmas day the half a dozen kids there wanted to race each other across the creek all day.
What I think are probably the key problems in getting kids on sups are lack of an entry point into the sport i.e. youth based program's and a lack of specific kids equipment. A scaled down 9'6" board and kids size paddle costs considerably less to produce than a 12'6" or 14' and a hell of a lot less than sports like motor cross and a mere fraction of my daughters ballet fees

They are also a lot easier than a big wide adult size board to learn the necessary skills.
Adapt there is talk of developing a junior squad locally that I and my kids are eager to get involved with. My hope is that others in other areas might do the same so that potentially we will see some future champs coming through the ranks that can compete and push each other and the sport forward. If the Olympics are on the radar then junior participation will be paramount.