I can highly recommend the Starboard Start, Rio, Go and Kombat Tuffskin, in that order of stability and beginner friendliness for adults. For Kids preteen with less emphasis on adults learning it would be Kombat Tuffskin first, especially if the adults were skilled enough to use the board in stronger winds.
Of at least equal importance is the modern learners rig. It makes a
huge difference for kids (and adults) learning.
Starboards kids rigs are great and reasonably priced but I have seen some others that are also good.
Pictures below of me teaching my mates daughters to sail on GO 170 (with side fins) and 2.5m kids rig. They were sailing back and forth almost immediately.
Claire up and going within 60 seconds
Jessie 60 seconds into first lesson
Nessie's first time
Nessie a few minutes later one handed
Jessie 20 min later powering on the Tandem
Seeing a begginner having fun is quite rewarding
My daughter during her first try at windsurfing at 9 or 10yo a few years ago with 2.5m kids sail and Starboard Kiddy board