If they are slim kids there is the Kadematic Junior for the 6 year old. Its a very small & light package but won't fit once they reach age 10 or so.
For the 9 year old I'd go for the Secumar Arkona midi, as its a bigger unit and they grow fast at that age.
But our kids are slim & fit, I have seen kids age 10 that can easily wear adult's lifejackets.
Get devices with D-Rings for tethers, and make them wear the tether depending on wind
And very important: bring some spare CO2 cylinders so the kids can try them in a bay to learn what happens when they fall into the water
, and know how to behave / swim with the jacket inflated.
I have to admit our kids wear them only when weather
is bad and they still want to be outside the cockpit
area. But a catamaran
is a different platform anyway
And when off the boat
, whatever) they don't wear a lifejacket at all. Ours are 10 & 11 now, and they swim way better than most adults. They always swim from the boat
to the beach, which can be anything from 50 to 400m. Of course only if there is no boat/dinghy traffic around.
Maybe in someone's view we are too careless, but I think we have a strong and healthy offspring that already knows a thing or two about dangers in the water