My girlfriend and I brought our 2 then 5-year-olds and they both did well. In fact, when we got out 1/2 up, my daughter wanted to keep going! No, its not easy, but if the kids are relatively athletic and not afraid of the water, they'll have a great time. Plus, there are men stationed all along the falls to help the kids (and the adults!) over the hard parts. They literally lifted me straight up in the air (WHEEEEEE!!!!).
Enjoy it - its definately a memory your kids will keep. Don't forget to bring a disposable waterproof camera with a wrist band so you can take lots of climbing pics.