If a field mouse is very young, they will only drink their mother's milk. If they are a bit older they will eat fruit, nuts, and seeds. If the baby is not with the mother, you can feed the mouse by sucking up some milk into a small dropper and while holding the mouse, let it suck from the open end of the dropper.
A baby field mouse can eat mouse food. Mouse food consists or grains, seeds, sometimes nuts and dried fruits and berries. You may also feed a baby field mouse alfalfa grass and yogurt bites made specifically for rodents.