Many thanks again! Ipcress, I bought the postcard in the UK. greenacres, thanks for link, great site! I still haven't found anything resembling these costumes though.
A few thoughts from the facebook group that talesof pointed me to: some confusion as to the gender of the older children but the concensus seems to be girls, given the dress style and centre-parting in the hair. Some suggestion the dress could be ethnic (fits with fancypants suggestion). Pendant's look like keys (as also suggested by mart). Interestly, someone suggested the dress of the older two might be related to freemasons/lodge dress, but others disagree given that they are thought to be girls.
So... still confusing! But fascinating and I will just keep my eyes open for anything similar.
I did come across a style of jewellery called 'lavalier' which holds some similarities to the pendants - a google search brings a few up. Would fit with the time period.