Post is meant to be both respectful and informative.
It depends on what you call a backpack. A backpack has large capacity in that it is designed for backwoods hikes. You can pack it with food, hygiene stuff, sleeping bag and so forth you need or several days, weeks, or more in the backwoods.
A daypack is smaller and designed for a one day excursion to something like a photographing trip or picnic in the woods. You're back on the same day heading home.
A knapsack, as far as I'm concerned is just another name for a daypack. BUT today they are used by students to haul books, laptop computers etc. Their capacity is not huge but you can often stuff a lot of little thing in side pockets and interior compartments.
In answer to your question, it is highly unlikely a backpack would be allowed. A knapsack shouldn't be a problem. BUT BUT BUT, be aware I based it on the fact I and Shirley have worn knapsacks usually with foodstuff and drinks (water, tea, etc) inside. We have never been asked to surrender them or leave for having them. If it helps, I also have a large fanny pack that I wear with the pouch in front of me to put things like my wallet, extra cash, game info booklets and sometimes extra drinks. I think there's one specifically designed for parents that i once saw at a mall several years ago. If I'm right you probably can find it on the Internet cheaper than mall price.