Hi Steve, there is one workaround for this requirement.
Since you have the Divi builder on your child sites, you can create this page that you need on all sites on just one site. Once you create it, go to your MainWP Dashboard and Search for pages on that site. When you get results, click the Edit action for the page and you will notice that MainWP will load it in classic editor where you will see a bunch of shortcodes created by the Divi.
Now, you can copy these shortcodes (all the page content) and go to the MainWP > Pages > Add New to create a new page on the rest of your sites. Paste the previously copied content from the page you created and publish it to the rest of your sites.
The Divi on the rest of your child sites will recognize shortcodes and load the correct layout. As per my testing, it worked correctly.
Since you have the Divi builder on your child sites, you can create this page that you need on all sites on just one site. Once you create it, go to your MainWP Dashboard and Search for pages on that site. When you get results, click the Edit action for the page and you will notice that MainWP will load it in the classic editor where you will see a bunch of shortcodes created by the Divi.
It might be worth of trying on a couple of sites for the start and see how it goes.