We've been taking our kids car camping since they were infants. My parents did the same with me. I took my son on his first backpacking trip about a year ago when he was 5 1/2. You can read the trip report here:

http://www.backpacking.net/forum/ubbthreads.php?ubb=showflat&Number=182172#Post182172

I took him this last Sept up into the Sierra. He had a blast both trips. He didn't have fun the whole time, but overall a very positive experience for him. I agree with the above about keeping the mileage low. Both trips I chose because of the low mileage and minimal elevation gain. I knew I would have multiple places to stop and kept track of his attitude to determine how far to go. The first trip we got about a mile down the trail. The second trip he made it about 3.5 miles with a couple hundred feet of elevation gain. He was going downhill fast at the end of that hike.

Kids adapt very well to being outdoors, problems come up when the parents are unable to adapt to what the kid is capable of.