Count on the comfort rating being 15 degrees higher than the manufacturer rating, unless you’re looking at a brand like Western Mountaineering or Feathered Friends. If you’re going to be hiking in a shortish distance and setting up a base camp there’s no need to go with 800+ fill power down, spend half the money on slightly larger and heavier 550-650FP bags. For the kids, consider women’s bags as they often have more insulation since women tend to sleep colder. They’re shorter, too, which means less air space to warm up.
REI gives R-values for sleeping pads, very handy for comparison.
Geartrade and Switchbackr sell used gear and returned stuff at decent markdowns. You have to know what you’re looking at, though, because private sale listings are often pretty short on info.