You get it wrong. I didn't create or use child template. Please read more about child templates in the docs.
Again, you don't have to create a template or use a child template. You can do everything in single post to /api/report.
In the limited way it looks like this:
If you would really want to use child templates. You need to first create a template using OData POST.
https://jsreport.net/learn/api#create