One of the great problems with Tab groups for a long time is that they hide Child Tabs from the UI — it can be very hard to find a Child tab if you have a number of windows and Tab Groups open at a given time.
Arguably the "simplest" solution would be to list Child Tabs in the Window menu, indented below their Tab Group parent, just as you see in the Tab Switcher app: https://apps.apple.com/au/app/tabs-switcher/id1406718335?mt=12
Having said that, it's true the Window menu length could get out of hand so an alternative might be to show children as a sub-menu under the parent Tab Group, but this would complicate the process of selecting the Tab Group without changing the active tab.
Thanks for any consideration — IMHO this is one of the key UX flaws that make Tab Group UX worse than it should be.