Bob,
Momma should take care of all of their needs for now but will push them away from Kitty Corner Cafe at about six weeks when they start biting her. That is the time they need to find new a Mommy and Daddy! It is okay to use canned kitty food, but once you start, that is what they will always prefer. Dry kitten chow softened by water at first, and once they get going on that, they will eat just like Mommy. However, many times a day you feed her and will subsidize their needs with your barn rats and snakes. I give mine a can of wet food for a treat now and then, but it is not so good for their teeth. They will need to get deworming and first shots at about 6 weeks old too!
As for your building a pen or something like that, good luck! I am sure you have always heard about the impossible task of herding cats! You are about to find out, but Mommy will keep tabs on them for a while and fight their battles for them, so hard as you may try, you are going to have problems keeping them confined to any place. A pen may work for a little while. Just shut your barn doors at night to keep the bigger critters out and hope Momma can keep them from going through the cracks! I am sure now that Mommy is okay with your holding them, she may be receptive to moving her home temporarily into your house.
You can try. If she does not like it, she will let you know.
Hope this helps!
Puppy