Hi:
Yes, you'll want to also map the parent ID.
Here's a sample form that has a mapping from a child group to a parent group - it will be the orders mapping - you can export that from the resource files to take a look at it in a text editor.
Here's a demo video that walks through the sample form above - the webinars are more quick demos than tutorials, this video should be more detailed.
In general, when mapping the child, you'll map the parent id to the appropriate column. If the child is a repeating group, this involves typing in the relative path to the parent. If you need help figuring out the relative path to the parent, feel free to post a picture of your main data source, and let me know the repeating child group for the mapping.
You'll submit the parent item using the qRules SubmitToSharePointList command and you'll use the /id parameter, so that the new parent gets its list item id returned to the node you've specified. Then you'll submit the child - since you have mapped that parent id field, the child will contain that info.
Let me know if you need additional detail or help mapping the parent id field.