TBH I'm not sure. There is a lot of winching which requires a bit of strength and jumping on and off boat when coming in. Also you will need to understand the concept of angles etc for tacking and jibing. Has your DC got experience of sailing or is s/he just caable in an every day sense of the word? There is a night sail as well. It would be worth emailing the RYA to ask.
For competent crew you need to be able to steer the boat to a course - could be a problem if rough weather - raise and lower the sails - can be physically demanding and again, difficult if rough weather. Has your dd/ds been sailing before? Is he/she used to the boat heeling over? You have to be confident moving about the boat forwards and aft, setting the sails for a particular course, tacking, tying a few particular sorts of knots, and other things which I can't remember off the top of my head! TBH, if there had been children on my CC course I would have been a bit narked - the RYA offer children's courses which may be more suitable. Probably best to discuss it with the course provider, though!
I think strength and size would be the main issues. I sailed as a kid and we were pretty comfortable with the jumping on and off and moving around in different weather. Intellectually none of it is particularly difficult and plenty of kids do knots, understanding how sails work etc, as part of dinghy sailing. However a yacht will be set up for adults. It may partly depend on the boat - a whizzy new yacht with nice winches would be easier than an old gaffer. In either case the child may not be able to see well enough to steer! Also it would be a bit weird to have an 8 year old as part of an adult class. Def check it out with the course provider.
There is a big difference between a bit of dinghy sailing and the sort of sailing required for CC, imo. My dd (only just 8) loves kayaking, but is utterly terrified when our boat (27 foot Westerley) heels over, so much so that we cannot go sailing with her any more as she hates it so much. She does want to do some dinghy sailing, though, so there is hope. There is an Introduction to Sailing course which may be more suitable for your ds on a big boat, as there is no pressure to do anything he doesn't want to do, but he will still get a certificate at the end of it!
Sorry, can I just ask, at what age did they start kayaking? We are living in kayaking paradise, and dh is keen to take up the sport. Our kids are 6 and 3. We were thinking of getting 2 double kayaks, so we could go with one child each. sorry for highjack, just wanted to pick your brains
DD started at 7 - she is only just big enough to handle a kayak herself now, but she would have been ok in a double. Double is a good idea anyway, in case they get tired. I'm not sure I would have taken her at 3 though! Whereabouts do you live? I like the idea of kayaking paradise!
From my experience this summer, my 11-year-old managed competent crew, but not my 8-yr-old. We did a week's sailing course with just us and the instructor and then a week on flotilla. DD2 (8) was very keen and learned a lot, but wasn't physically strong enough for the winching in and out of sails and this was a non-tidal course in the flat calm Ionian. She got a lot out of it though and was the fender knot champ by a long way.
And whyever not, Seeker? It is much more attractive than a lot of the names parents choose in these degenerate times. Lady Chardonnay Palliser, anyone? Where are you sailing, btw? I envy you, I would love to do another sailing holiday.