If you just have them confined in a room, do a kitten check and make sure there are no cords or just things that they could get hurt by. They also still will want to go back to their nest to sleep. You can begin introducing solids as well as the litterbox now, it won't cause any harm to do either. Some may take it to the litterbox and food quickly, while others may take longer.
I have heard of using the pie tins as well, it just needs to be low enough that the kittens can easily crawl in and out. Sometimes giving too much freedom or too much space too soon with result in accidents outside of the litterbox.
If that begins, I would advise putting them in a smaller room, like a bathroom, or possibly confining them to a smaller area, such as a puppy pen, so that they can get more accustomed to the litterbox, before giving too much freedom.
I would also recommend setting up different scratching surfaces, like sisal rope and corregated cardboard, so Momma can begin to teach them to scratch in proper places. I would also begin getting them used to nail trimmings soon, so when they go to their new homes, their owners can easily trim the nails.
Good luck, your babies are growing up!!
It goes quickly, sometimes too quickly!!