IDhunter,
I recommend you buy a book called "Horses, Hitches and Rocky Trails" by Joe Back. It will set you back about 7 bucks, but its full of very good advice on horse packing.
The only advice I would add is use your horses a lot. Even if you are just packing hay bales, get them used to hauling stuff while you get used to using them. Having fun with horses is all about trust. There isnt any trust if you never work together with em. Have fun.
I've only been to Challis once (last spring), but river access is good. Get a public lands map and you'll find lots of access. Some of the property owners are happy to let you on to fish if you just ask. By all means take your family. I saw a few families fishing on the river bank. Theres a nice campground not far upstream from Challis if thats your thing.