Someone must have two PKU genes in order to have PKU.
You and your husband do not have PKU, but you are what are called carriers. Carriers have one PKU gene and one normal gene. Because your child got a PKU gene from you and a PKU gene from your husband, they have a total of two PKU genes and therefore they have PKU.
If you have another child you have a 1/4 or 25% chance of having a PKU child (you each pass on a PKU gene), a 1/2 or 50% chance that your child will be a carrier like you and your husband (one PKU gene from one parent and a normal gene from the other parent), and a 1/4 or 25% chance that they will receive two normal genes and not be a carrier or have PKU.
I hope this helps! Let me know if you have any other questions.