Is it possible to embed a New Item Form within another Form?

@jacallan .....I don't think this is possible. If you want parent/child relations between two lists then you need to use Repeating Sections and Nintex workflows to manipulate these repeating section.

Below are the step by step instructions (I've copied my response on similar repeating section post..link to that post ):

Instead of using UDA:

1. Simply create a MLT (with plain text) column in a list. Plain text type is very important since it's going to store XML code of your repeating section
2. On the form, connect your repeating section to MLT column you've created in step # 1. You can remove this MLT column from your views in the list.
3. Now, create a workflow and:
   1. Use Query XML and use the MLT column from step # 1 as a XML source
   2. Use collection variable to store all the different values of your controls you've put in repeating section
   3. Use one of the collection for "For Loop" and get the index also
   4. Use collection operation for other collections using index
   5. Now in for loop, you have all the values need for each repeating section row, use workflow to maniputlate as per your requiremnt
4. Now when you create the child item, for each child item you also provide the item ID of the parent item
5. This could be useful for updating the child item (if someone modified recurring section's existing rows) and if there are new item then just simply create new item in child list
6. You can create views in child items to group them by parent IDs

Let me know if this works for you.

It WORKED!!! Wow thnks. I was pleasantly surprised... 

short life

@joelroberts ....also when you create the child item.....you can get the item id of that child item and store the same in your recurring section of the parent item. Make sure this control stays disable all the time (I prefer to use 1<2 as a rule to always disable controls). 

Now, before creating the child item put a condition and check if child item control in the repeating section is empty or not.....if it's empty only then create the child item. This is to make sure we don't create duplicate child items in child list.

Also could you please mark my post as accepted solution?

I changed tactics to using a workflow & custom start form at the parent to collect the child data in form fields and passing those to the workflow to create the child item.

I'm struggling a bit with creating the hyperlink the user will use within the parent form to initiate the workflow, but the rest of it was pretty simply.

@jacallan ....could you be please more specific? Also check if the workflow is allowed to trigger manually or not? If it is then usually there is an workflow option on each item properties page.