Pumphouse or Radium to Rancho is a good bet. There are several good camps on that section, our favorite is 'Island'. It's located just below Yarmony Rapid on river left. There are 2 class III rapids; Needles Eye between Pump and Radium and Yarmony between Radium and Rancho. Sorry I dont have the river map in front of me and I don't remember the exact river miles. The BLM has good info including a map you can download and print on their site, the map is hard to read when printed.
Both rapids are straight foreward but at higher flows have holes that you should be able to miss but could flip a boat. This section is great for family trips but I would wait for the levels to come down before going with my kid. You will see many other boats but most are doing day trips so competition for camps is not bad. The only real bummer is the trains, they do run at night.
If you want more info or are interested in having another family join you let me know. We have a 6 year old daughter and a 3 year old lab, always looking for other familys to raft with.