Most people overpack when going on a trip because they think that they will wear all the clothes they are bringing. When you first pack, take about one third of your clothes and put them back in your drawer, because no one in Europe is going to remember that you wore that shirt two days ago in a different city.
While traveling, you will be too tired to go out every night, so also put back the all the fancy clothes that you intended on bringing. If you do plan on going out every once in a while, you should pack one nice shirt and one pair of nice pants or a skirt, and that’s it. Don’t forget to bring nice shoes, as well, because unlike the United States, bouncers won’t let you into many clubs or bars if you're just wearing sneakers.
To maximize space, consider buying a compression bag to squish your bulky sleeping back into a smaller size. At the top of the pack, above the heavier items, place other gear you might need to access during the day. Layered clothes, a map, flashlight, sunscreen, toilet paper, medical supplies, and a first aid kit are some examples. You'll also want to keep your lunch in the top of the pack as well. Otherwise, you'll end up unpacking half of your bag to access this afternoon meal.
For multi-day packs containing hoops and straps, it's also possible to secure items to the outside of the bag. Tent poles, rope, and sleeping pads are two good examples. Again, just make sure the pack is properly balanced. Don't put too much weight on the pack's exterior or you'll find yourself being pulled backwards from its weight. This will result in you leaning forward to compensate, which leads to poor posture and fatigue.
As far as having enough room to bring souvenirs home from Europe or wherever you are traveling, try not to buy a lot of things because they will hold you down. The souvenirs will put a lot more weight on your back than you really need and, more than likely, will break when you are throwing your backpack around. My advice is to have enough space for one or two things that you really like, but don’t go overboard.
If you are helping your child pack backpacks, you could follow these guidelines. Lay the heaviest items in the backpack first so they will be closest to the child's back. Arrange books and supplies so they don't move around in the backpack. Check the child's backpack to make sure the items they are carrying are absolutely necessary. When the backpack is too heavy, the child may hand carry a book, lunchbox or other items to lighten the load.
Just remember two things, "Pack it light. Wear it right."