In regards to the USA; Churchill did send a personal Telegram to the President to ask him for ships to deliver food to Australia. You can read the message below:

I am seriously concerned about the food situation in India….Last year we had a grievous famine in Bengal through which at least 700,000 people died. This year there is a good crop of rice, but we are faced with an acute shortage of wheat, aggravated by unprecedented storms….By cutting down military shipments and other means, I have been able to arrange for 350,000 tons of wheat to be shipped to India from Australia during the first nine months of 1944. This is the shortest haul. I cannot see how to do more.

I have had much hesitation in asking you to add to the great assistance you are giving us with shipping but a satisfactory situation in India is of such vital importance to the success of our joint plans against the Japanese that I am impelled to ask you to consider a special allocation of ships to carry wheat to India from Australia….We have the wheat (in Australia) but we lack the ships. I have resisted for some time the Viceroy’s request that I should ask you for your help, but… I am no longer justified in not asking for your help.

Unfortunately Roosevelt was unable to comply because he was advised by his chief of staff that diverting ship to take the Wheat from Australia to India would have an adverse effect on US military operations. I can't find the whole transcript but it is listed among the churchill parpers

During WW2 the Axis powers engaged in Unrestricted submarine warfare which meant that the it wasn't just a case of having the amount of merchant ships but the allies would have also needed to divert military ships to act as escorts to protect them from potential attack, so It is somewhat understandable that the USA would be reluctant to assist given the importance of the pacific campaign. I should note that Eleanor Roosevelt did raise money for the famine via the 'All India Womens Conference'

To some what conclude. Churchill didn't necessarily disallow the USA to send to famished india and in fact he did request the USA's help in delivering food from Australia, which he believed to be the shortest haul and therefore the safest from submarine attacks. I do not know why he delayed asking Roosevelt in the first place but It would probably a mix of not prioritizing the Famine over the war and perhaps already knowing that it would be unlikely for the USA to divert war resources to a British colony, especially after the USA had already done so much in regards to supplying the UK throughout the conflict.

Hopefully someone can weight in on the Canadian part of your question.