Interesting. "Cox's Sandpiper" was first noted in Australia in the early 1980s and was later proved to be a hybrid between male Pectoral Sandpiper and female Curlew Sandpiper.
Quite what a juv "Cox's Sandpiper" looks like c.f. a juv Curlew Sandpiper is something I'm not sure about. But I would be interested to know why this is not just a juv Curlew Sandpiper.
David