Have kids help you make homemade trail mix, also known among backpackers as gorp. To make basic gorp, you'll need 1 cup each of salted peanuts, raisins, and M&Ms. Mix it all in a large bowl. Add other ingredients (sunflower seeds, cashews, granola) as you and your children desire. Portion out.
Read more about day hiking with children in the guidebook Hikes with Tykes.