I think the key with kids is to start them early and challenge them so they look forward to stretching their limits.
I started Nathan backpacking when he was about five and by the time we did Whitney in
2001, he was an old hand at it at age 12.
Of course, he's now 20 and in college, so there aren't as many opportunities for us but he will occasionally deign to hike with ol' Dad, as we did last year at
Humphreys Peak in Arizona.
Keep it fun, remember you're going to hike at
their pace when they're young, but don't be afraid to make 'em stretch their legs once in a while.