Why don't you just take a bottle of evian with you? if they confiscate (which I can't imagine they would), the plane can supply bottled water.
This may not be what you would use at home, but for the sake of a couple of feeds it will be fine, and it's not as though your babies are newborns any longer so their digestive systems are more mature.
Plus, if you are only carrying the powder (rather than pre-made), takes less space and weight in your hand luggage!
This is what I have done in the past.