Is it worth doing with a 6 year old?
I would definitely get the booklet. It doesn't force you to do anything that you weren't going to do, but it will provide an incentive for a youngster. Assuming you're not just all about the rides, whatever trails or treks you have planned to do there will be a "badge" he/she can collect. And it's a nice keepsake for them. You can also continue it on your next visit.
If you're a park commando, maybe skip it. If you plan to look around and do the jungle trek, gorilla falls and see some of the animal trails then do it!
My 7 year old really got into it. It kept him interested on the safari and while wandering around the animal paths.
Our DD was 5.5. We did a few of the stations and she loved it.
My son is 7 and he loves it! The cast members are great with the kids! Try a few stops out to gauge your 6 year old's interest. Pick up a Wilderness Explorers book just after the Oasis. A perfect starting point would be to take the train to the Conservation Station. There is a stop shortly after you get off the train. It's called : Habitat Badge; Find animal friends in your own backyard. Should be page 21 unless they have updated the book since May 2017. The kids have to locate 'creatures' found in their backyard. Then, there are a 5 badges inside the conservation station. They are pages 22-26. You can get all of them within a half hour or less. The Conservation Station is air conditioned and usually not crowded, so your 6 year old can relax or burn off a little steam. Not sure if they are into Doc McStuffin's but she can be found inside the CS too. Sometimes cast members will bring out a few animals for the kids to view. Enjoy exploring
You won't likely finish it in one trip - it took my son three trips to do it all - but it is so much fun for them, and a nice opportunity for you to rest a moment too.