Just make it simple please!!!
My philosophy: Start them now!
Gear: Get them a hydration pack such as a camelpak. We have this one but any 1/5L small one will do for your kid as young as three and up to 12! Yes! The kids can put their own snacks in the pack such as: apple slices, trail mix, veggie sticks, dates, and/or a healthy bar.
Hiking Trails: Check out this book for the whole family ~ Coast to Cactus: The Canyoneer Trail Guide to San Diego Outdoors (page 363 has a great 3 mile hike for as young as 6 yrs called the Azalea Glen Loop Trail)
Character Building: Snacktime is at the halfway point or depending on timing “the top”