We have a model that starts a multi instance subprocess. We would like to be able to get the status of the individual subprocess instances associated via the REST API. How can we accomplish this?
Have you considered using the process instance get list API  to get the list of sub-process instances? You can use a filter based on say businessKey, or you could filter based on the parent process instance id by passing it across in the superProcessInstanceId filter parameter.
Thus once you have the set of sub-process Ids, you could use the get API  to get the detailed status for each instance.
 https://docs.camunda.org/manual/7.7/reference/rest/process-instance/get-query/ https://docs.camunda.org/manual/7.7/reference/rest/process-instance/get/
Thanks for the suggestion. We will try it out and let you know how it works for our use case.