Churchill was not Prime Minister when the MRP was announced or when it went into effect. He wasn't even in the government at all. He was in Parliament, but mostly an exile due to his bellicose views. It was the war that forced the Conservative government to take him in, and he didn't become Prime Minister until after France was invaded, well after the carve-up of Poland occurred. In fact, the main benefit he brought to the office when he was elected Prime Minister was that he bore no responsibility whatsoever for the disaster the UK now found itself in.
Roosevelt was running an officially neutral country 10,000 miles away. There was no CIA, and Europe was (officially) at peace. I doubt he knew much of anything about it before the pact was announced.
Now the MRP had both public and private sections. The public part was publicly announced roughly two weeks before the invasion of Poland. This was essentially a non-aggression (against each other) pact. The exact extent and contents of the various spheres of influence in Eastern Europe in the pact were in secret (not publicised) clauses, which only became publicly known after the war. However, it was quite clear to everybody at the time that some kind of agreement had been struck, because the Soviet army marched into and took possession of eastern Poland while the German invasion was taking place. In the absence of some kind of prior agreement that would clearly be a hostile act.
If you were to guess at what the agreement was based on what their two armies did, you'd have gotten pretty dang close to the terms.
Note that the only difference here of any significance is that Russia didn't take Finland. That wasn't for lack of trying though. The only place you'd be liable to guess very wrong would be Lithuania vs. Central Poland. My understanding was that the German panzers took the extra territory as part of their encirclement tactics during the invasion, so Stalin decided to let them keep it in exchange for Lithuania (to complete his matched set of Baltic States, so he could set up hotels I guess).
So yes, certainly the UK (the USA was still not in the war yet) knew there had been deals both public and private between the Soviets and Germans when they allied themselves with the Soviets after the German invasion of Russia. It didn't much matter at that point. If either wanted to survive independently at that point they had to work together. If you read Churchill's memoirs of the war, he doesn't even particularly blame them for doing it.