The listing says 'tiny' size, but these pants weren't worn ankle-length. And WWI isn't exactly right; I don't know the cutoff date, but my father's unit wore these pants as late as 1928...
The uniform hung in the attic chifferobe as late as 1955-56...Tunic, pantaloons, puttees, cover, some other pieces I can't remember...
And the girl in the eBay auction is awfully skinny...