Well Anakin knew he had a son for years and couldn't find out who it is over the years.
Did he? He was told he killed her in his rage, not that she gave birth then died. And during her funeral she still looked pregnant; they were hiding the evidence she gave birth.
That was a hilariously skimpy attempt at a cover-up. For starters, Anakin shouldn't have been so completely gullible that he'd take Palps' word for it. (Of course he was gullible about everything else, but that's a larger issue of bad characterization.)
And imagine the number of people who must have known the truth about Padme - doesn't Naboo have a coroner? How about her family members, friends of the family, political hangers-on, that sort of secret would get around. There's no way it could stay a secret for 20 years. It wouldn't stay secret for 20 minutes.
Compared with that, keeping Luke "secret" with a Jedi on Tatooine under the name of Skywalker and Leia "secret" as an adopted daughter of one of the very few trusted allies the Jedi had left, are far easier to believe.
You...are...evil!For example, during the trench scene in Episode IV, as Vader bears down on Luke's X-Wing but just can't get a lock, Vader's line will be changed to: "This one has a high midochlorian count!"
