I would say that Rafiki's Planet Watch is a good use of time, particularly if you are otherwise planning to spend a full day at DAK. My nine-year old's dream job is to be a vet there, so we'll be heading back there in hopes of catching a procedure of some sort. Beyond that, it's basically a petting zoo, but we've also seen some pretty unique characters back there that we don't typically see anywhere else -- Pocahontas, Jiminy Cricket, Rafiki and others.
On the flip side, if you've got expectations that this will be a half-day for you, then you would not miss a whole lot.
We're planning to ride Dinosaur for the first time this summer. I've tried to get my nine and 12-year olds to watch a
YouTube video of it. At the moment, they are OK with it and plan to ride, but a buddy of mine rode it with his two little girls and said they became permanently attached to his kidneys during the ride! I guess it's all a matter of knowing what you're getting in that ride before you get on it!
My kids have always loved the safari, EE (at least in the last couple of years when they started riding it, so your eight-year old might enjoy it), as well as the Lion King and Finding Nemo shows. They are not big fans of the Bug's Life show under the Tree of Life, but I may force them to watch the movie again, so maybe they'll change their minds. Interestingly, they also think that Kali River Rapids stinks -- way overhyped and way too short of a ride. They don't even want to waste a FP on it. Either way, this time, they are most excited about trying out the new Wilderness Explorers activity. I'm guessing that one will take us all over the park, looking for stuff, so we're going ahead and planning a full day at DAK and will just take it easy.