A little over a year ago, I was working at a science center where one of the mice (Snoopy) had a litter. Another mouse was due to be giving birth any day and all of Snoopy's babies except one had been fed to the snake that lived at the Center. I went back one day when we were closing and I saw this cute, 6 day old baby mouse that was covered in brown fur and had a long tail with a black line down the back of it. I fell in love with her.
I took mommy and baby home and we watched this baby mouse (we named her Remy) grow from an infant to an adult herself. Mommy and baby were together every single day and slept next to each other every single night. At the end of March, Snoopy started looking sickly and less than a week later she passed away. We took Remy to the vet and found out that they both had gotten mites from the bedding we were using. Needless to say, we switched bedding and now Remy is mite free. I still feel guilty that we didn't realize that Snoopy was sick too, but we just didn't know.
Both mice are fancy mice so they aren't the kind that you find outside. None-the-less, I would take in any small animal that I knew would need help.
OP, in my book you did the right thing. Please try to update us when you find out what is going on with the baby.