My Gf has a child with PKU, apart from a special diet he is perfectly fine.
They just can not have anything with alot of protein in it, meat fish chicken, dairy, there is so much they can not have, but you can find alot they can have too,from the general supermarket. They also buy some special food for him from a (o not sure what the place is called now) but it is pretty expensive, however the government does supplement her for the costs.