I've taken my oldest three out when they were 5-6 years old for their first trip.

The trip was chosen based on it being easy (3 mi in/3 mi out), interesting (waterfalls, water to play in, rocks to climb on) and was done on THEIR pace.

Examples:
http://www.hikingupward.com/GWNF/ColdMountain/ (went with friends. Less good than the other two)
http://www.hikingupward.com/JNF/AppleOrchardFalls/ (this one was great and often repeated, despite the stinging nettles)
http://www.hikingupward.com/GWNF/MountPleasant/ (rocks, views, a boy scout troop and a bear)

They walked in front so that I could keep an eye on them, and they could see. We stopped for insects, bushes, trees, and anything else they were interested in. As more kids joined in, the youngest stayed in front and we walked in age order (youngest first) for the most part.

Food was chosen based on THEIR taste. I loathe mac&cheese, but I've sure eaten a lot of it because it made them happy. I brought extra. I brought candy.

I kept their load LIGHT. No more than 10% of body weight. When starting out, this meant that they had a kid sized camelbak, rain gear, a whistle, trail mix, and a small stuffed toy of their choice (kept in a plastic bag).

I learned to carry crocs in my pack for the kids when they were around water. Painfully. A kid got wet, and then cut their bare feet on my watch. Thank God my wife is forgiving. Just carry the little crocs and you'll be thankful you did if playing around a watersource might be on the agenda.

We also tended to go when it was WARM. Spring, Summer, and Fall in Virginia. 45 degree bags were usually overkill, which was good.

Oh... all of mine had car camped with my wife and I a couple of times before they tried backpacking.

Every child had fun learning to pee and poop in the woods. Definite highlight of the trips. Also, if you are a man taking daughters you can't spend too much time on this. I got distracted when my 3rd child started coming and she ended up WET. It wasn't a big deal in July in Virginia, especially with a creek to play in later.

Oh... the camelbak was a great idea too because they'd suck on it absently and stay hydrated. Of course, this made for more pee breaks but that isn't a problem.

BTW, everyone who spoke first on this issue REALLY helped me out when I started to take my kids about 4 years ago. I joined this site at least in part because of the "backpacking with kids" article on the main page.

Have fun!!!