Keep them in a small area for now, preferably one thats easily cleaned. As above, use non-clumping litter and low trays. I put an extra one out as the mother cats I've fostered have all used a hooded litter tray with a fairly high entrance. They will copy her - in my experience they are desparate to use her larger tray! The same with food. Feed her in the area where the kittens are and they will start copying her when they are ready. It can start with paddling and swimming in her food and be very messy, and if she has a lot of milk and nurses them freely they might be 6 weeks or so before you see them eating food.
Whatever else you do, don't let the mother out until she is spayed. She can easily get pregnant again while she is nursing her kittens.