Sweet. We are about 5 years behind you. Cant wait to take my two little ones on a more serious hike. They hiked the "grove of the patriarchs" this last weekend while camping at Mt. Rainier......very cool.By age 4 or 5 they can do stuff like Little Si, Rattlesnake Ledge, or even Lake 22. Start 'em young - bring snacks and drinks and they can do more than you'd think!Stephen's got it right. When my boy was 4, we took him on a hike up to Blue Lake near MSH. Two+ miles and 1700 VF of gain, and that little whippersnapper hiked every bit of it w/o so much as one complaint. Enjoyed the hell out of himself. Like the man said, snax and drinks and they can go almost forever.
Also, for boys at least, it seems to be endlessly amusing to be allowed to pee outside. And when you come to a lake, let them throw rocks into it for awhile. That is usually the high light of these hikes for them. :-)