I haven't had a child with a PWS, but I am a child with a facial PWS. I have 2 brothers, and neither of them have birthmarks of any sort.
As for laser treatments, I strongly reccomend them. At the age of 15, my birthmark is almost completely gone because of the treatments I've been recieving since a young age. True, it can be a somewhat painful, but it's over with relatively soon, and the results are worth it. Also, with new technologies in the lasers, I'm looking forward to the treatment I have tomorrow because I barely feel it.