The key here is not to carry too much weight.
But most doctors and physical therapists recommend
that kids carry no more than 10% to 15% of their
body weight in their packs. [1]
The way to not carry so much weight is to intentionally purchase a backpack that only holds a small amount of stuff. This constraint will make it hard for you to overload it and cause back pain.
Another tip is to try out various types of backpacks. There are padding ones, like you mention, or ones with a hip belt (which offloads the weight from your back onto your hips) [4], or even side-satchel (messenger) bags. My preference has always been for messenger bags.
My favourite bags (over 8 years old and still in use) are:
* Mountain Equipment Co-op (Canadian company) Kilobyte Computer Backpack [2]
* Waterfield Designs Cargo Laptop Messenger Bag [3]. At $230, it's expensive, but the build quality is so high that the amortized cost less than $30/year and it's still going strong (no frays, zippers still work, etc).
[1] http://kidshealth.org/parent/firstaid_safe/outdoor/backpack....
[2] This model is no longer sold, but it looks like this: http://shopping.canoe.ca/images/detail/40578000/40578375.jpg
[3] http://www.sfbags.com/collections/shoulder-messenger-bags/pr...
[4] Although hip-belt backpacks are quite large, they are amazing things. I went backpacking with one (i.e. lots of weight, lots of walking) and the hip belt saved my belt, and even spread out my vertebrae enough to get rid of old back injuries. Of course, I'm no doctor, but that was my experience with them. For everyday use, a hip-belt on a small backpack may be either hard to find or overkill for your needs.