will xs fit a size 6 (above average sized 5 year old)

not likely: we actually got the 5/6 XS for our above average 4 year old, and it fits her nicely. I don't think she is as big as the average 6yo.

you might find that the 7/8 S is alright for a large 5yo if a perfect fit isn't necessary; it's a comfortable casual summer play dress that my girls pull on and off over their suits, or change into to lounge around on hot afternoons...
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