I'm working on the assumption that 1"X50 means they are 1" wide and the 50 is the length (though I hope it's not inches as has been indicated, cm makes much more sense).
So the 57's will be 7cm longer than the 50's you have (that's 2.7, nearly 3, inches). I'd call that significant if you are so short that you are riding with your current leathers on their shortest or next to shortest hole. Though if you can't find 50's or shorter you can punch more holes in the 57's and they'll work fine; you'll just have lots of extra leather.
I'm 5ft and actually don't ride with kids leathers, but I could probably get away with it. I've been lucky in that my local tack shop was willing to order me reasonably priced ($50-60ish) "in between" (I'd guess youth, or extremely short adult) leathers. They are out there, but they are hard to find! :) I currently have 3 sets stockpiled (one on the saddle used and two as backup so I don't have to deal with finding them anytime soon ;) ).