Hi there,
Its not the backpack but the weight of the books in it that could have negative effect on your kid's health. At present, considering the nature of the education the children have to carry a lot of books,notebooks into their backpacks from a very tender age. But too much of weight could lead to many complications, such as neck pain, pain in spine, shoulder pain. Prolong exposure to such pains could lead to spoliosis, tear of spinial chord, breathing problems due to bad posture.
Hope this helps
Take care.