Klingon_Jedi's idea for Ep III ("Tell the children of their true parantage we must not, Obi-Wan. Better they believe a different truth than the terrible one that is. Until the time is right, unaware they must be.") seems much better.
If you really don't want Ben to lie, then TheDemonHunter's idea (cut "how did my father die") could be made to work, or you could just have Ben not answer that question.
Most importantly, from the "saga" point of view, IT DOESN'T MATTER that we know Vader is Luke's father from the PT and he doesn't. The revelation still works if it's only to Luke, and it's arguably more interesting for the audience to be party to information that he doesn't have, wondering IF and WHEN he'll find out.