What a lovely collection of ponies!
Being in a country which sometimes I wonder knows if sunlight exists I havent encountered this kind of worry too much...but your example of fading is pretty drastic over just a couple of weeks
As others have said I would recommend either UV protection film on the windows or thicker curtains...3M is a top notch brand for vinyl and print materials, but there are others which will be good but not as expensive...and in the meantime make sure anyone sensitive to sunlight [i.e. anyone with fading pink hair] takes a couple of weeks vacation out of any sunlight!