Airport Security and What to Expect
The 3-1-1 rule doesn't apply to breast milk, regardless of whether mom is with baby. Also, you can request for a hand check on your breast milk, which I have always done, since I'd like my milk to be as radiation free as possible.
Pack your milk bags in a small cooler bag, but bring an extra gallon zip lock in case. If the TSA agents ask you to remove your milkbags from the cooler to put your cooler pack in the x-ray machine, you can use your large zip lock to hold all of your milk bags. This way, you won't be putting your bags directly on the dirty bins.
Every checkpoint has their own SOP on how to handle people that opt out, or carry liquids. This can range from having your milk bags swabbed, getting a pat down, or getting your entire bag and contents swabbed inside and out. If you're traveling in and out of the same checkpoint frequently, you can ask the agents what their procedure is, so you know what to expect everytime you pass through that checkpoint. Knowing that you might be be at security longer than usual can help with managing travel time, and ease some travel anxiety
In case you are met with any trouble, or feel that either you or your milk is being handled in a manner that is uncomfortable with you, don't be afraid to ask for a supervisor. All the TSA agents and supervisors I have met are extremely respectful and helpful, and can do wonders to allay your concerns over how your breast milk is handled.