I have waiting for four years now and my children (17 and 12) have never asked.
If/when asked, I decided I will tell them their father was unfaithful. I really do not foresee this question coming up until they are in adulthood and can really think about it more.
I really think my DS (17) knows....he has made comments about how "NICE" dad was to various woman during the M.
Mind you....they were always the most upstanding woman - you know....like the nice lady with the house-arrest anklet at the bar where he had my DS at.