i usually have a crew i ride with and focus on them, but as you're mulling around while people hike and what not, of course you're going to be checking out the other riders. i don't think it's a problem if you take kids pics, but if they start looking paranoid (especially if you're younger) tell them what's up and offer to shoot them over through email. i've never had someone say no to photos, if anything, they'll be more amped. besides, it's a good way to get yourself out there.