Hellobee Boards

Since it's an international flight, check with the plane about dietary restrictions. I would think they would allow it to be stored with them as it is medically necessary and cow milk is not an option. No harm in calling and checking!

I think a thermo would be tricker with the liquid restrictions.

I agree that ice packs might be tricky to get through security; if you want a similar effect, I think you can use frozen peas or other food though.
I second the suggestion of talking to the airline in advance and, if possible, get some agreement in writing. I'd also have a doctor's note with you about Charlie's allergy and the medical necessity of bringing your own milk, just in case you get hassled for bringing a liquid.
Any chance hemp milk comes in juice box sizes so you could only open one container at a time?

I agree that the flight attendants would prob be willing to keep it in the fridge for you.

Could you bring some dry ice? Dry ice is allowed on planes. Then just ask for ice either on the plane or at one of the food places in the airport. Then you can keep the cooler at your seat instead of asking them to put it in their fridge.

You probably only need a little chunk of dry ice since you need it to be cold and not frozen.

Well, I know you wanted to pack light. This isn't exactly light packing if you have to bring dry ice and a soft-sided cooler.

you'd have to double check to see if these are allowed, but these lunch packs are awesome:
http://www.amazon.com/PackIt-Freezable-Lunch-Bag-Forget/dp/B004SIX9YY/ref=sr_1_1?ie=UTF8&qid=1343698431&sr=8-1&keywords=freezer+pack+lunch+box

you keep it in your freezer and it has built in gel packs in the lining so it keeps the contents cold without additional packs.

i will say, though, that international flight attendants are usually super accommodating, especially towards families. they'd probably store it for you in the refrigerator without hesitation!

Can you find it in single serving packages at all? That's how I transport milk on flights. Do you have to bring enough with you for the duration of your trip? The usual recommendations are to only bring enough for your travels and I wouldn't feel comfortable transporting that in checked luggage, so you might need a bigger exception than you think.

I wouldn't count on the FA's being able to store anything for you. They usually don't have extra room for that kind of stuff, so I'd call ahead and ask. As an example, on one flight I was on, there was no microwave to heat anything up.

I'd bring a thermos and a soft sided cooler that you can fold up. Also bring ziplocks and ask for ice. Most restaurants will give it to you no problem and you can replenish it on the plane easily.

I think @Andrea option is the easiest.

This bag is freezable and doesnt require an icepack. I bought it after someone on this forum suggested it. Its great.

http://www.amazon.com/PackIt-Freezable-Lunch-Bag-Polka/dp/B0049CVWCK/ref=sr_1_1?ie=UTF8&qid=1343741843&sr=8-1&keywords=freeze+it+cooler+bag

ETA I see Mrs cowgirl already suggested it here lol

I would bring a thermos. We always have a thermos when we travel to keep things cold or warm. Whatever you do, don't bring individual size milk with you on carry on. This probably varies with different countries and different airports though. I didn't have this problem living abroad, my airport now (I think JFK is the same) makes me open all containers that contain liquids so they can check the contents. Is there a powder version of the milk?