Get a second opinion
I would seek a second opinion from a pediatric dermatologist. Like you, I would try to look at the bigger picture and say, "this is just a thumb, not a more obvious place, so just be thankful". At the same time, you are caring for your baby, and not living someone else's birthmark. It can't hurt to get a second opinion from a specialist. If it is diagnosed as a PWS, you can decide how to proceed. Based on my experience, it took a while to navigate the system so it's best to start now. For me, it took over a month to get an appointment with the specialist (and that was with the benefit of having a mutual friend; would have been two months without the contact). Then, we waited 8 weeks for our first laser treatment - purely due to limited appointment availability. Our insurance has covered significant amounts of the treatment cost because the PWS is on my son's face. I can't speak to how they would deal with an area (i.e. the thumb) that is less apt to cause psychological/emotional distress. I don't think you've missed or are on the cusp of missing the window of opportunity. However, I am an advocate of early intervention simply because I believe, based on the literature I've read, that the results are more effective.