Thanks for all the suggestions. PeteCz I didnt even see #65, thats a good one to keep in mind.
Jim- A friend of mine suggested Twentymile for the fishing, and I have wanted to explore a new part of the park also. Another vote for #65, Bearpen Branch, Ill have to try this one next, but it is on the far side of the park for me (Im in Knoxville).
Crockett - Thanks for that link, I didnt know what the stream looked like at #18, and from the pictures it looks very kid friendly. That had been first on my list before, I think I might end up there after all.
Grouseman - Thanks for the suggestion on the campsite. As for the hike being too tough, this three year old is as big as most five y.o. and has been hiking since he could walk good. He's done several 2 to 3 mile hikes before. Here is a pic of us heading down the Stiffknee trail on the way to Slickrock (which is a tough trail and about 3 miles, Eli , the 3 y.o is on the left). We never made it, but that was due to running into 2 yellow jacket nests on the trail about a quarter mile before the creek. They didnt complain once other than that.