No cow's milk, the mouse would just get diarrhea from the lactose and dehydrate.

Keeping a mouse pink alive is next to impossible. Euthanize it and stick it in the freezer, or take it back to the pet store. It is cruel to try and keep such a tiny thing alive without much "How-To" knowledge.

Your Ball Python would most likely not even recognize such a tiny mouse as food, especially if it has been away from the mother mouse for a few days(less likely to be as active, not as healthy, could be dehydrated, near death, etc).

Now small adult, already weaned, mice are the proper size for younger Balls, and are more apt to be recognized as a food source. They are easy to keep alive with a simple cage set up that includes a water bottle, bedding(kiln dried pine or aspen shavings), and some food(whole grain cereals, dry pasta, oatmeal, granola, etc, or lab blocks if you have them).