Hey Tom - I boated the Fraser with you a few years back. I've done several overnight trips on the upper C. May be doing it next weekend depending on weather...
Pumphouse to Catamount makes a great 1 night rafting trip. More scenic than the lower section, and the camping is better. Windy point camp has several sites with grills, picnic benches, lots of trees, and even has a pit toilet. Drawback is people can drive to it so not as "rustic." Forgot the name of the camp above it but there are several old cabins that the kids may like to check out. 1,000 - 3,000 would be good for the kids. You'll likely do fine at any level, but I think lower water would be better for the kids first time unless you have a good crew and want to take chances with swims. I'm assuming you are rowing?
Below Catamount the camps are few and far between. Burns rodeo rapid and twin bridges can be tricky. I ran it over 10,000 cfs so maybe they are less of an issue at lower water? There will be a lot less people here - you may even have the river to yourselves, but I like the upper section better.
Either way you should bring earplugs and prepare for the train to wake everyone up several times per night.