RRG covers a lot of ground. Most of the approaches will involve some uphill hiking. There are some fairly short approaches, but a half-mile is probably average for the crags I've climbed at. Most belays have some flat ground, but some are tighter than others.
Your best bet may be to get the Ellington guidebook; it'll generally give you a good idea about the length of the approach. You might check redriverclimbing.com for an online guide.
Is it crazy? I saw a couple packing in their 20 lb baby and crash pad this fall. I consider that dedication. If your three year old will hike a half mile, it probably wouldn't be too bad. That said, you might think twice about Muir Valley, which is a pretty popular area, as most of the approaches are more like a mile, and they can be kind of steep.