There are two campsites down in the valley, Camping Pirinenc and Camping Moli Serradell. I haven't stayed in either but from driving past Moli Serradell looks a bit more 'in nature'.
The very first wall you get to at Montgrony 'main cliff' is perfect for kids: flat and wide open - see http://imgur.com/a/VBq3C . There are a handful of good routes 7a-7c on the wall. Beyond that it gets a bit rockier, and you have to keep an eye out, but still manageable. The upper tier crags (6s) are not really suitable for kids at the base of the crag, as you're in a fairly dense thicket of bushes and trees.