We had the genetic test when my daughter was 6 months old (here, it is done for free).
They do a test to the little one, to know which two of the 400 mutations is the child carrying (one of each parent). A mutation is a small change to the DNA of the parents that can cause the deficiency of the enzyme, when it is present to both parents.
There are almost 400 known mutations, each one having a different severity. So, from what I understand, if you do the genetic test you can
i) predict the severity of the disorder. (This is not always the case, as doctors say that the only way is to test your child's tolerance by diet) and
ii) predict when you're pregnant weather the second child will have the PKU, doing an amnioparacentesis.
You decide if this knoweledge is usefull to you or not.