If you have at least one ounce of breast milk, you are allowed to bring on board a frozen gel pack in the US. So put a bottle with 1.5 ounces of milk in with the ice pack. Also, bring along a bunch of plastic ziploc baggies just in case a TSA agent didn't know the rules and seizes your ice pack, so that you can at least get crushed ice.
I've done this before and when I got to the security screening, I told the agent that I had breastmilk and a freezer pack. She didn't even want to see it! She told me to put it back in my bag, and put it through the x-ray machine. No problems getting through security.
If I were you, I would print out the regulations for bringing breastmilk aboard. Here are the two relevant links:
Flying with breastmilk:http://www.tsa.dhs.gov/travelers/air...n/formula.shtm
Flying with medication (breastmilk is considered medication) which
needs a frozen gel pack for preservation:http://www.tsa.dhs.gov/travelers/air...eds/index.shtm
Note that both regulations state that you must declare what you are
carrying PRIOR to screening. That means you need to get the
attention of an agent and tell them you are carrying on board
breastmilk BEFORE you put your bag through the x-ray machine.
Otherwise, the regulation implies, they can take your stuff. So just declare it in a loud voice, and they will likely pass you right through without even looking at your stuff. If they ask why you are bringing milk on an outbound flight, its because you had to pump at the airport
I'd ditto what star*mora said about flying with frozen - its going to shorten the life of your MM, and if you get delayed, it might melt even with the frozen pack. Hope that helps!