redirect to a child theme based on the town of Internet users in france with the verification of the ip address

A parent and a child theme haven’t been designed to do this.
In your case you need to create template file for your tags (cities) i.e: single-paris.php, and load it with template_redirect filter, according to the request.
Another way to achieve it, is to add a body class that will call another css values, but you’ll have less possibilities.

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