Yes, well FDR's reputation before he was elected president was that of a dimwitted scion of a powerful family.
Yes, it was like taking candy from a baby for Churchill.
FDR was actually a pretty shrewd canny fellow. You don't win the presidential elections four times in a row by being naive or stupid, especially since he was a cripple (from polio) for most of that period. He managed to outmaneuver his political opponents over a long period of time.