I don't think that the pink & white dress looks dated; I love cap sleeves. Didn't even realize for one bit that the dress is from a whole other decade and even now knowing it is, it doesn't look "off" to me.
I believe they keep all those clothes for posterity primarily. Other kids & adults now wear the clothes too, which is perfectly fine and nice to see, but I'm sure it's been kept for the future, many exhibits can be done with those clothes in loads of places. "Fashion of royalty", "queen's & princesses fashion", "the evolution of fashion", "a look at the year xxxx" "designer xy's work throughout the decades" etc etc. I love clothing from the past (the V&A had a marvellous display) and these will be equally valuable one day (partly they are already).