How do you intend to load your 5 year old?

Mine carries about a liter of water, rain gear, a whistle, and gorp (10 essentials that are age appropriate).

Also, consider using a hydration bladder for them; it keeps them drinking b/c they'll sip absently on the tube.

Further, consider the difference in hiking speed between your 15 year old and your 5 year old. Your group can only move as fast as your slowest member. Your test hikes should be instructive, as should your daily walking on the farm (and to the store, and so one).

Sincerely

Steadman