Why does the IF commission the Wiggins to have a 3rd child? What does this say about the Formic Threat?

Ender's parents are allowed to have a "third" because their first two children were exceptional. Unfortunately, Peter was too violent; Valentine too soft. Thus, they are given a waiver in the hopes that the third child will be the genius they're waiting for, and that his demeanor will come in somewhere between his siblings. This shows the Formic Threat is more threatening than they're letting on.