Hi Snizzerbizzler,
Maybe you can add child forum directly in the forum webpage of the portal.
1.Navigate to the forums parent page within the portal.

2.On the portal inline editing toolbar, select New, then select Child forum.

3.Specify values for the fields provided, and then select Save.

From the screenshot, you can notice that parent page and forum template and thread forum are import, so you need check them in the portal management when your child forum not display.
The following is default configuration in customer self-service portal, you can refer it:
(1) forum template and thread forum.

(2)Parent page and it's page template.

Regards,
Leah Ju
Please mark as verified if the answer is helpful. Welcome to join hot discussions in Dynamics 365 Forums.