It's time for your kids to go back to school. I have a child in 3rd Grade and a Pre-K student. Rather than buying a brand new backpack for my daughter's school year, she is going to re-use one of her old backpacks from her previous grade. The backpack is in perfectly good condition and does not need to be replaced.
She had 5 backpacks stored at the top of her closet. I understand her not wanting to use a Dora backpack (too young for her age), but she can use her backpack with the flowers on it.
My advice to you is to re-use your kids backpacks. My second piece of advise is to buy backpacks that can carry over from year to year. I recommend buying a durable backpack and one that is not theme-specific. Your kids will grow in and out of themes (Dora, Hanna Montana, Little Einsteins, Hello Kitty, etc.). Buy a backpack that is a generic design or color that can be used for more than one use.