What a great pony [and gorgeous cat!] - Starflower is a great pony.
I certainly agree the first bet is a magic eraser and if no luck with that then definetely try sun fading.
I am going to be a bit of a negative nancy on this one though. We bat the "highligher" phrase around a lot. I find it odd that children of the 80s had such regular access to pink highlighter pens - most notably never any colour other than pink [dont think I had highlighter pens in my house back then let alone would have been allowed access to something which has a smell like that as a 4 year old]. Also the pink marks are never left in the place you would expect a little girl to add "pink" - i.e. cheeks or lips or eyes - always very random. Personally I am reasonably confident a lot of these marks are a chemical break down and unfortunately if that is the case then they may not fade out.