One of these backpacks are on correctly, and one is not.
The one that is not could be causing back pain for your child, so make sure that your child's backpack is fitted properly.
The arm straps should be pulled so that the full backpack fits securely on the back, it should not be sagging down past the butt. Once it is fitted correctly, it should ease their back pain. If they continue to complain, then it might be time to get out of the seated position and get them moving as they need to work on strengthening their core.
With kids, exercise should be fun and be a part of their play time; not "working out". So make it a game!
Go to the park and have them climb things, swing, and practice using the monkey bars. This will help them to strengthen their muscles in a more natural way.
Some other exercises they can do for fun are:
Hopping on one leg
Children love to emulate and spend time with YOU! So don't just send them outside to play on their own, get outside and play with them. You will not only build better relationships with your kids, but you will also be showing them that exercise is fun; and while you are at it, it will be providing you with a great workout as well!