Do you have any experience in helping with nutrition for children with PKU?
Gosh, I don't. I do know that children can still partially breastfeed, with Phe levels being monitored.
Since it's common for medical establishments to be behind on the newest studies, here's a link to what appears to be a good summary of current advances and knowledge: http://www.nutritionandmetabolism.com/content/9/1/7 (let me know if you can't see the full-text here).
Good luck to you and your little one. The silver lining is that a PKU diet tends to be very heart-healthy (but see the fatty acid concerns in the paper I referenced).