In addition to the other answers and regarding the safety of the frozen milk, you need to keep it frozen so it won't get spoiled, ask the airlines if they can provide some "dry ice" packs for you. It is not allowed to bring it yourself as it is considered to be a "dangerous goods" item, but the airlines can authorize it to be onboard (it is actually avaiable in almost all flights with the meals (if available) in the chillers in case the chillers failed". If they agree to provide it then you will be sure the milk will reach it's destination safely as the dry ice will make the milk frozen for many hours.
Alternative option if you managed to get it onboard, ask the crew to place it in the chillers for you, eventhough it is not allowed per almost all airlines' policies but crew will always understand such cases and try to help :)
P.S. I have first hand experience with this as a cabin crew member.