JayB--Twentymile isn't a bad choice--but--the best fishing is lower down and it's unquestionably too rough for kids that age.
There's another campsite upstream (roughly two miles) on Pretty Hollow which is a lovely place--no horse campsite is lovely, IMO.
A better bet might be to go to the Bearpen Branch campsite on Noland (that's the name although the actual Bearpen Branch is a few hundred yards farther up the trail). It is just over a mile upstream from the trailhead, the walk is a easy and leisurely one, the fishing is decent+, there's a nice long, still pool which lies right under the road at the point where you turn off for the campsite which would be great for playing (oddly enough, it is a side channel, because the creek splits here, but over the years I've caught some fine fish there, including a dandy brown just three weeks ago).
One other thought. That's where I took my daughter (more years ago than I really care to think about), and we had a grand old time all of us (my wife was along) still remember. Of course one thing which helped make it memorable was a (insert colloquial word here for vented posterior regions) of a seasonal ranger who wanted me to take frying fish out of the pan for him to measure them. I had had a dickens of a time getting a fire started, since it had rained for two days straight, and I was cooking over an open fire. Also, the five trout all measure between nine and eleven inches, so it wasn't ever close. I got so irritated I told him to just write me up if it came to that, but I wasn't ruining my supper. I also did something I probably shouldn't have done. I mentioned the regional ranger, whom I knew well since he attended my parents' church, and said that if he did write a ticket I was going to pull the smallest of the fish from the pan, carefully wrap it in foil, and the two of us were going to hike out and have a visit with that ranger. That, basically was the end of the conversation. First time I had thought of that in years, although my daughter got a very bad impression of rangers thanks to one (use another impermissible synonym for the end opposite one's pie hole--now I hope Paula doesn't chastise me, but as the foregoing suggests, the guy flat out frosted my grits.
Wherever you go, I trust you'll have a grand time, and kudos for passing on a wonderful legacy.