Usually they want you to get a bigger backpack so the larger library books will fit in it.
I just went to Target and got a Thomas the Tank backpack for my kids when they started preschool. I used that backpack and then made a big deal and got a new backpack when they started kindergarten.
The Lands End backpacks will last a few years, but they are more than the backpacks at Kohls and Target. For the older kids, I always made sure their backpack had a spot for a water bottle. They always got the Lands End ones and they lasted about 3 or 4 years.
I would NOT get the ones on the wheels. Many schools discourage that and even when they are older, they don't fit in the lockers or are a pain with the bus or getting in or out of a car.
So for preschool, I would say get anything she wants. They all have zippers and easy openings. Kohls has some on sale this week as well as Target.