Just did it last week with my 4 and 5 year old.
Book as far as you can in advance. It fills up fast. And pricey - at $30 per child. There are seldom no shows or cancellations and they only run in three days a week. If its the least bit chilly, dress your kids appropriately, the wind whips around Bay Lake.
The kids enjoyed it. We got to the GF marina at about 9:00 for our 9:30 cruise. They checked in the kids and put them in lifejackets and bandannas. There were more girls than boys and most of the kids were in the 4 to 8 age range. At 9:30 they paired up, loaded them on the boat and took off. 24 kids, 3 adults. (Be warned, you do sign away your rights to sue if anything happens to your kid on the cruise. Isn't an issue for us, and I'm sure there aren't many people who take their little pirates away instead of sign, but it might make someone think twice about putting their kids on that boat).
From there its pretty fuzzy what happened. We know they went straight over to the Poly marina (cause we went to the Kona Cafe for brunch and saw them debark). At the Poly the apparently had to find the other half of the map. .....
And it gets even fuzzier. We aren't sure what happened, but my kids talk about "pirates" and a "booby trap" and finding the treasure in the "woods" (WL?). They pulled in at 11:30, with peanut butter sandwiches, juice boxes, rice krispy treats and gummi bears in a bag (we ate waiting for the monorail) and as they left the boat were handed their share of the "booty" - Mostly the stuff you get at a kids birthday party - rings, mardi gras necklaces, a superball shaped like an eye, whistles.....
They had a good time and I'm glad we did it.
I wouldn't recommend it for a first timer unless you are going to be at Disney for a long time or already planning on going again soon - however, by your next trip your eight year old would be too old to enjoy it. We were there seven nights and made it into MGM only to see Fantasmic, didn't see everything we wanted to at AK, missed much of Epcot - but we did do things like Blizzard Beach, the Pirate Cruise, and a long big sit down dinner every night .... if you are going for the parks, it will take an entire morning (prime park time) and you may leave wishing you'd been able to see more of the parks.