If you're flying out of Newark, I seem to remember some of the shops open around 6 am, so you should be able to pick something up. But generally you're allowed to travel with liquids for a baby (please see TSA webside for more details)- we've been allowed through security with juice bottles, and other liquids for a baby. One thought might be to pour the milk directly into a bottle or sippy cup, so there is no question it is for a baby. I put all my baby liquids, etc., into a ziplock and put it through separately, letting security know.
DS and I have flown all over, no trouble, even through Newark on Continental. That way, you're not counting on the airline having something you need - because sometimes they run out (esp. if there are long flight delays), or they had delays and are not able to stock the plane with beverages for shorter flights.