You should get a box that has a lip or one that is covered to prevent litter from being kicked out. If they try and tip over their box, then you’ll want to get clamps and fasten it to the pen/cage.
Your bunnies look young and so they are just being young. Just like puppies can tear up furniture, and cats can scratch up stuff, bunnies can chew chew chew. Does the bedding have any perfumey smell. You may want to try different beddings to see what they will actually use, not chew. Do they really eat it, or do you find little pieces everywhere. IF they are eating it, then you want to watch their poops as this can cause health problems.
I find a cheap way to replace bedding is to go to a fabric store and buy yards of fabric – usually fleese like, flannel and/or some soft cotton.
Do they have any other chew toys to distract them? You can try and offer something that will keep them busy – like a cardboard toilet paper roll filled with hay. Put some pellets deep inside of the hay (inside of the roll) so they have to work at getting to it. This may help distract them from the bedding.