Aloha anotherKevin,
I take my girls hiking along with other parents and their kids. My kids are 2 and 5. Kids can surprise you.
Just start mellow and progress from there. We take our kids walking pretty regularly, both around the neighborhood and to our local parks.
The girls first real hike was to Manoa Falls. It is about 1 1/2 miles. Hike is pretty much uphill to the waterfall and back downhill. Elevation gain was about 500 feet so pretty modest. But both girls made it up to the top under their own power. My five year old also made it down herself while the two year fell asleep at the top as we were waiting for the last mom and her son to catch up. I couldn't beleive we had to wait about 1/2 an hour. Then I had to carry my daughter down.
Here a link that has some pictures of Manoa Falls:
http://www.hawaiiweb.com/html/hiking/manoa_falls_trail.htmlOur next hike was Aiea Loop Trail. It is just shy of five miles and I think with more elevation gains and losses than Mano Falls. My two year old wanted to be carried for most of the hike, but my five year old and her buddies completed the hike.
Here is a link:
http://www.hawaiiweb.com/html/hiking/aiea_loop.htmlOf course, I was very proud of both my kids as well as the other children.
The hiking site is probably not the best around, but you get the picture.
Just take your kids out and have fun and let them explore and have fun too. But, of course, go equipped.