Third Grade Biology

Mouse (n.)
A small rodent that typically has a pointed snout, relatively large ears and eyes, and a long tail

Rodent (n.)
A gnawing mammal of an order that includes rats, mice, squirrels, hamsters, porcupines, and their relatives, distinguished by strong constantly growing incisors and no canine teeth.
 
Mammal (n.)
A warm-blooded vertebrate animal of a class that is distinguished by the possession of hair or fur, the secretion of milk by females for the nourishment of the young, and (typically) the birth of live young.


Since all mice are rodents and all rodents are mammals then all mice are mammals. Since young mammals are nourished by their mother's milk and since all mice are mammals then young mice are nourished by their mother's milk.

That's correct, wild and domesticated mice breast feed. If you find baby mice in your back yard with no mother in sight, please leave them where they are. Their mother still has to eat and may have gone to forage for food or water. Even if the mother never returns, pet stores don't carry a suitable milk for the young mice. You can try cat formula, but the chances of the mice surviving are pretty slim. Chances get a marginally better if you take the mice to a local wildlife rescue. But if you insist that I don't know what I'm talking about and insist that they can eat adult mouse food they're going to die.