I am using Jira Cloud instance v3 API to create an issue that has a field with child objects.
For e.g :

I am able to create an issue but the child object is not getting created inside
customfield_10039 field as shown below.

At the same time when I am replacing the children node which accepts array of child with the child node which accepts a single record. The child node created succesfully.
For e.g :
Child node inside the issue : 
But the metadata for the create issue has the children node. So, Which one is the appropriate field "
child" or "
children" for creating issue with child objects??