Ugh. I'm so sorry to hear that. I am familiar with "PR". My mother-in-law had it when she was young with small children and she said it was all over her body including her hands. Hers itched horribly and all she could do was wait for it to pass. Apparently it isn't supposed to spread to areas where the sun touches...(mostly found on the trunk, upper arms, uppper legs... depending on the time of year) and you are supposed to get it once and then never again. At least she has only ever had it once.
Now with that being said, I have had it probably 6 or more times, never severe and always on the trunk and occasionally on the back of my upper thigh. Mine has never itched and does stick around for a few weeks. It starts as a small dot that grows bigger (they call that the mother cell or main cell) and then smaller ones appear around it. Eventually, they may appear to grow large enough to join together. It is really harmless and goes away in a few weeks but I hate when that happens.
I had it when I was pregnant with my first child and freaked out that she would start getting it. No sign yet - 6 years later. Sadly, I saw a spot appear a few days ago on my side so I'm waiting to see if more appear. Each time I get it, I notice it covers a smaller area so I get annoyed but not upset.
I've seen a doctor about it at least 2-3 times and each time they dismiss it as a nuisance that will go away. Honestly, that is all it turns out to be... but it is VERY annoying. No one ever knows how you get it or what you can do to avoid it. I've never been told to put on a cream, except if it itches which mine never does.
I feel bad for you because it is your child. Try not to get stressed about it and try not to dwell on it. It will look much worse before it goes away. Finally one day, they will appear a little lighter. Then even lighter the next day. Then the skin will just look a little rough or the color will be uneven.. then they'll be gone and you'll breath a deep sigh of relief.
Hang in there. Feel free to contact me directly if you have any other questions.