Good for you! I am sure the mama rat and her little ones are happy you are taking care of them.
I have heard people talk about giving some egg to the mama rat before and I am sure it is a good idea, but unfortunately, I have never had ratties that were nursing with pups before (I have always had boy rats, so I know nothing about your topic, sorry!)
BUT, I know there are some great people on this forum that would have a lot of info to give you, like Lilspaz, who takes in rescue rats all the time and has a lot of experience with them. Perhaps you could send a private message to Lilspaz and ask her your questions, in case she is not on the forum in the next day or two to see your question?
In the meantime, doing an internet search will bring up lots of reliable rat care sites like the Rat and Mouse Association, etc, and you can just type in your search words for nutrition for nursing rat and babies?
You can also type in search words on this forum to find past posts on the topic as well.
Let us know how it goes with Mom and her kids! I would love to see photos.