Here are some thoughts as to trying to get around the restrictions.
You could trying putting bottles with milk in a small insulated bag or container. I've read the restrictions, and they say formula or breast milk. Frankly, I don't think a TSA
screener is going to quesiton you as to whether you have cow's milk.
There's always trying to getting milk inside the secure zone and having your kids drink it before boarding, assuming that you are on a short flight. If you're flying a longer flight, though, you could be facing the problem of no milk after the first round of beverage service.
You could also call you airline and ask if the agent could put some short of notation for your flight that you will have small children would appreciate extra milk being put aboad by catering.
Finally, since you should be allowed to pre-board, speak with an F/A about either getting milk after takeoff, or at least having some set aside, so that you don't have to worry about the beverage cart being empty by the time it gets to you.
I do understand your concern. We usually carry water with us, when our 3-year old travels, since the beverage service always starts at the other end of the cabin. My wife is planning to ask the F/A, while I'm installing the car seat, if we could get some water as soon as the F/As start preparing for beverage service.