Hi Monique!
My daughters are 11 and 14, similar to the ages of your niece and nephew. I was looking for something unique to experience on our trip just a few weeks ago. I found the Wild Africa Trek in Disney's Animal Kingdom Park. It was absolutely incredible.
We arrived for our 3 hour tour and checked in right next to the Kilimanjaro Safaris. Our group of 12 were suited up in harnesses and vests, given name tags and water bottles, and encouraged to use sunscreen and insect repellent. We were also fitted with earpieces so that we could hear our guides at all times.
Once inside the forest off of the Pangani Forest Exploration Trail, we were able to get up close with the hippos and even talk to one of the hippo researchers. We then walked across a couple of rickety bridges (while being harnessed in, of course!), including walking precariously over the crocodiles! All the while, our guide was snapping photos of all of our amazing moments. These photos are included in the cost and are accessed once you are back to a computer.
We then headed out in a private safari vehicle to experience Kilimanjaro Safaris like never before. We stopped to "ooh" and "aah" over the two baby giraffes and then again to see the lions, who were actually awake in the middle of the day. We then veered off the regular path to a covered porch area where we dined on exquisite African cuisine with an amazing view of the flamingos, elephants, and Harambe Wildlife Reserve. Even my picky eaters thought it was delicious!
After lunch, we returned to reality after what did NOT seem like 3 hours! If you can work this experience into your budget and space is available, I would highly recommend it!
Have a wonderful Walt Disney World vacation!
Tracy V