I know that I'm a bit late on this one.
However, I thought I would post, in case anyone else stumbles onto this page, looking for a solution to a similar problem.
The examples provided by "HeathDurrett" and "Firedog" both contain legitimate JSON Syntax.
The "Firedog" example contains a single group of Objects (key/value pairs) and therefore, square-brackets are not required.
However, the "HeathDurrett" example, contains Arrays (multiple groups of Objects), which require the use of square-brackets.
I should also Note that, in this case, the value order, in regards to the "url" and "name" pairs, do not matter.
If "HeathDurrett" wanted to use his example, within Group Policy, he would just need to compress his JSON Notation into a Single Line (I also prefer to remove any/all unnecessary spaces, such as those between the key/value pairs), as follows.
[{"toplevel_name":"MyCompany"},{"url":"http://intranet.mycompany.com/","name":"MyCompany Intranet"},{"name":"RDWeb Portals","children":[{"url":"https://internal.mycompnay.com/internalresource01.html","name":"Internal1"},{"url":"https://internal.mycompnay.com/internalresource01.html","name":"Internal2"},{"url":"https://internal.mycompnay.com/internalresource01.html","name":"Internal3"},{"url":"https://internal.mycompnay.com/internalresource01.html","name":"Internal4"},{"url":"https://internal.mycompnay.com/internalresource01.html","name":"Internal5"}]}]
If you want to Test your JSON Notation, before creating a GPO, you can do so via the Windows Registry Editor.
Simply Open your Text Editor of choice, Paste in the Code Example below (or your own JSON Notation) and Save it as a .REG File.
[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Edge]
"ManagedFavorites"="[{\"toplevel_name\":\"MyCompany\"},{\"url\":\"http://intranet.mycompany.com/\",\"name\":\"MyCompany Intranet\"},{\"name\":\"RDWeb Portals\",\"children\":[{\"url\":\"https://internal.mycompnay.com/internalresource01.html\",\"name\":\"Internal1\"},{\"url\":\"https://internal.mycompnay.com/internalresource01.html\",\"name\":\"Internal2\"},{\"url\":\"https://internal.mycompnay.com/internalresource01.html\",\"name\":\"Internal3\"},{\"url\":\"https://internal.mycompnay.com/internalresource01.html\",\"name\":\"Internal4\"},{\"url\":\"https://internal.mycompnay.com/internalresource01.html\",\"name\":\"Internal5\"}]}]"
NOTE:
This is the exact same Registry Key/Value that will be Added, when the resulting GPO, is applied.
When you Double-Click on the .REG File, it should Import the associated Registry Key.
Finally, Open Microsoft Edge, to see that your Managed Favorites have been Added to the Favorites Bar.
Feel free to respond with any Questions or Feedback. I'll be happy to provide my assistance, if needed.
Lastly, as I'm sure you've figured out, already, the reason why the response from "Firedog" couldn't be Set as the Answer, is because he Submitted his response as a "Comment", as opposed to an "Answer".