I would recommend getting rockhounding books for each state you will visit. The "Rockhounding (statename)" series tend to be pretty good, also "Gem Trails of (statename)". Generally most free collecting sites will be on BLM land or specific areas of parks (Crystal Peak in CA, Garnet Hill in eastern NV for example).
That being said, some places are easy to collect and other not so much. Some will require 4wd and a longish side trip. Some minerals can be hard to spot the first time out as well.
I can provide details on some areas in Nevada, mostly the western part of the state Reno, Las Vegas, Lovelock, eastern Sierra Nevada mountains and west to about Austin NV. If you're going to be passing through any of those areas send me a PM and I can provide GPS coordinates and information.
Lastly I would recommend trying to make sure the collection location is good for kids. Easy pickin's can get them interested in the beginning, hard stuff like fossil hunting (which has other legal issues on BLM land) might put them off the entire idea.
Good luck! Wife and I enjoy Rockhounding quite a bit, often our camping is just to provide a base camp.
2015 RAM 1500 4x4 5.7, 3.93
2013 Econ 16RB TT