Just another opinion.....
I would hesitate to tether children down in anything but extreme conditions. I think it's better to get them accustomed to moving around on the boat and learn how to cope with the motion. Ten year-olds are most certainly old enough to start helping out with the sailing as well.
On any reasonably equipped boat there should be enough security in the cockpit and within the lifelines for all but the youngest children. They must learn the first 3 rules of sailing at an early age:
#1 Stay on the boat
#2 Stay on the boat
#3 - Stay on the *&%$# boat.
If they learn to rely on a harness, the first time you decide it's not necessary is the day they'll go over the side. We sailed with our kids from "days old", only tethered them in once about age 4 or 5 in a lumpy crossing - in all the time we cruised with these kids not once did anyone go overboard by accident. They grew up learning "one hand for the task, one hand for you".