I have never bought a backpack from TCP but I sent my niece to kinergarten last year and her backpack was never full. You might have one small binder, a lunch box, a library book, and a jacket inside and the cutesy ones work just fine for that. If you are going to get the cute backpacks vs a more expensive one, you will probably have to chage it out within a year or two anyway because tastes change, and it should last until then unless the school is sending the kinders home with novels and a set of encyclopedias!
If you plan on using this backpack for multiple years, into the upper grades when the kids have to carry their own text books, then spending the money seems wise, otherwise you can just wait to spend the money in a few years.