Baby squirrels are fed on: green beans, peas, and small pieces of corn on the cob, lettuce, cucumbers, cauliflower, mushrooms, apples and strawberries. They can also feed on dry monkey biscuits or formula from commercial stores.
1 Additional Answer
Baby squirrels eat their mothers' milk, or a specialized formula, until they get their teeth. This usually lasts three or four months, after birth. After they finish nursing, they eat seeds, nuts, fruits, leaves, and other things that they can find in the wild. You can find more information here: http://www.boobooinc.com/sq/faq.html#eat