In the book Ender's Game why is ender's parents allowed to have a third?

Question

In the book Ender's Game why is ender's parents allowed to have a third? Please help me

Answer 1

Because they already produced 2 geniuses and were hopeful prospects to produce the third (Ender) which they did.  

Answer 2

Well, we learn by reading the book that the I.F. are searching for someone who can save the world from the Buggers. They wanted someone who had Peter's excellent skill and ability as well as some of Valentine's compassion. The figured that if the Wiggens had a third child, then they would have just such a child.