We got back from WDW 3 weeks ago (sigh...) and we did the Wildlife Discovery eXcursion (we were staying concierge at the Grand Floridian) - me, DH, and twins DS8. There were only 9 in the group but I think the max is 14. Our boys loved it, as did the other child in the group who wsa 5. I suspect he didn't benefit from all the knowledge being shared by our 2 guides and driver.
The 3 kids were given an animal spotting guide, clipped to a handy dandy lapdesk, crayons and binoculars. We weren't disappointed - we spotted every animal on the guide, and then some!
I think the $$ was worth it for our family, not so sure about the younger child. We learned lots and had plenty of time for pictures, asking questions etc. The guides (and other guests) were very patient and welcoming with the kids' questions, and the kids were so entertained they didn't mind the more in-depth discussions at other times.
Unfortunately, the dad of the young boy wasn't feeling at all well and at one point had to exit the vehicle to "toss his cookies." The guides said it was a first for them and the first time they had ever heard of a guest setting foot off the vehicle. I really felt for the guy, though.
I'll admit to feeling very spoiled when we would pull off the main road and savour our views of the animals as the regular Safari vehicles would pass by, 10+ would pass as we were still gazing away. I don't know how long the regular safari ride lasts, but our discovery excursion was >1 hour.
Hope this helps!