At least in canon, there doesn't seem to be any evidence that Owen Lars and Beru Whitesun had biological offspring of their own.
They are a more or less traditional, married, farming couple (albeit moisture farming). Given the many duties that we see Luke performing in A New Hope, it seems that they could use all the help they can get, and — if you compare to farming communities on Earth — one way to get that help is by having a large family. Granted, they did have many droids as @Himarm points out in comments, and so that might replace the need for people. Also, more mouths to feed out in the desert may not be a good thing, as @terdon mentions.
Still, all of this raises a question:
Were Owen and Beru capable of conceiving? If so, did they make a conscious decision to have no children of their own apart from raising Luke?