After I was born my parents along with my two older siblings became vegetarians. They were vegetarians until I was nine. If you decide to change your diet this would be your best bet. Eating a low protein diet would not be safe for the non PKU members in your family. The only reason those of us who have PKU can safely eat a low protein diet is because we have our formulas to give us the nutrients that we need.
If you decide to support your little one by becoming a vegetarian that is great. If not it won't be the end of the world. Eventually your little one will grow up and realize that others eat differently than he/she does. The best thing you can do is teach her that everyone has something, whether it is diabetes, food allergies etc, and PKU is just his/her thing. No two people eat exactly the same.