I have a wild mouse and baby-how do I release them?

While cleaning one of the large tubs stored in my garage we came across an extremely agitated mouse. She was hopping and flailing everywhere so I caught her with plans to let her go in our near by field. But after catching her and looking through the rest of the box I found one lone baby.I suspect she had been stuck in there a while and may have eaten the others in order to survive. I can't let her go because she'll take off without it- And I can't watch it starve to death or kill either of them so now what? I have her and baby in the same tub and have been feeding her liver,cheese bread and a capful of water everyday, How long before the baby is old enough to let them go?Baby has fur and is about as long as the end on a womans pinky finger from the last nuckle Maybe 1/2 inch thats guessing large-I have handled it only once to move the rest of the things out of the box? Any Help??!! She is taking care of it well but I don't disturb them often, I have had them for 2 days. I have 5 cats- I can't have mice for pets and they are wild.