It really depends on the excursion. You'll have to look at all the snorkling ones listed and pick one that allows 6 YO's. Some of them do, some of them don't. All of the snorkling excursions that I've seen ( I could be wrong) don't allow anyone under the age of 5. I believe this is the cutoff for the more tame snorkling adventures, and NO ONE under the age of 5 can even attend.
DCL won't let someone under 5 (or the age listed) even go on the trip, I was told it's because of insurance issues. So, if you want to take your 6 yo on a snorkling adventure, you'll have to leave the 4yo behind, either in the clubs or with another parent.
As to instruction, most instruction on a snorkling trip, regardless of from DCL or independent boat, is just telling you how to use the mask and fins during the ride to the snorkling spot. Usually it's only approx 5 - 10 minute talk and you're on your own pretty much (meaning sometimes someone (usually one person) from the boat jumps out if it's a guided tour, but if you're just swimming around in a confined area they don't even get off the boat). Unless your 6 yo is a VERY strong swimmer, I'd want to be in the water, since I'd be afraid that first he/she would swim off and not understand to go that far. Also, sometimes when snorkling, there are stronger currents out in the ocean that wouldn't be aproblem for an adult, but a 6yo might get pushed and have problems swimming in such a current, since it might be too strong for them.