For day hiking purposes with children, adults will want to get themselves an internal frame backpack, in which the frame giving the pack its shape is inside the pack’s fabric so it’s not exposed to nature. Such frames usually are lightweight and comfortable. External frames have the frame outside the pack, so they are exposed to the elements. They are excellent for long hikes into the backcountry when you must carry heavy loads.
When purchasing a backpack, you’ll first need to find the right size. Measure your back from the vertebrae at the neck’s base to the point in the small of your back that is level with the top of the hipbones and then examine the backpack tags to see which one matches your length. Next, look for padded straps and a padded waist belt for comfort and sewed joints to ensure sturdiness.
Always fit yourself for a backpack before buying it. Select one that rides comfortably on your back, that doesn’t chafe, and that doesn’t stress the shoulders and pull on your neck. When fitting it, load it lightly. Stores usually have pillows and sacks for doing this. Adjust the straps. The shoulder straps should be snug but allow your arms to move freely while the hip belt should rest comfortably on your hipbones.
Ensure that your backpack will have enough space to carry the essentials (more on this later) and that there is extra space to haul those items that younger kids decide they’re not going to carry anymore. Lighter packs mean less stress on knees, back, shoulders and neck. They also mean you can walk more quickly to safety.
A couple of final thoughts: If buying a backpack secondhand for either you or a child, make sure the material isn’t wearing thin and that the straps and joints are still in good shape. Also, if a backpack has metal zippers, consider replacing the pulls with cloth ribbons. The metal can get cold in wet weather and low temps.
Read more about day hiking with children in my Hikes with Tykes guidebooks.