We flew Gatwick to Toronto and they had me drink his milk, his water in his sippy and I brought along some drinkable yogurts and they made me open one and drink a bit, which ticked me off as they have a foil wrapper, too messy to reseal so I drank it there and then and DS had the other one on the plane. In Toronto they didn't even bother to make me taste test liquids. They never even bothered about any of the food for him at either airport. The flight attendants will be able to open your cartons for you or you could bring powder in zippy bags and bottles of water or the flight attendants will give you water and even warm it up for if need be.
Aisling x