This is only possible with a custom function that overrides the results after post processing. I've put together a possible solution for you. This function will go through the results and if finds child terms for the term, then it will replace them, otherwise leaves the original.
Please note that this is a customization request, which is not part of the support process, so I cannot guarantee this will work in all cases, always as excpected. Always have a back-up of everything, before editing any of your site fiels.
Place it into the functions.php file in your theme directory (copy from line 3):
It gave me positive results on a test environment, let's hope it will work for you too.Best,
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