You know your own kids, so rely on that. My experience backpacking with kids is that they can go as far as they want to. Motivation is the limitation, not energy or endurance, so training is not the answer. If they want to do it, they can walk you into the ground at that age, but if they don't want to , trying to make them is a drag for everyone and will make it the last trip they want to go on for a long time. The ideal thing is to pick an area where you can vary the length of the days as you go - so if they are loving it and want to go further, you can do it. And if their motivation is not up to it, you can stick to shorter days. I'm trying to get my kids to love backpacking, and it has worked great with my older boy - he's 14 and wants to do longer and longer hikes now. We started with very short trips so that it's fun the first time out.