There's not very much bakground for sixpence on the internet
I know a few facts if these are helpful:
The story was first published in 1921 but republished in 1924 in 'Something Childish And Other Stories'
Sixpence was excluded from 'The Garden Party' collection because Katherine Mansfield thought it too sentimental and the story appeared in it's place.
Anyway here's a link to a short paragraph I found. Not very informative but just a condensed summary/analysis