json files in the parent theme, if they are included by adding a json load point do not need t be synced into the database to work. If you don’t want these field groups to be synced then they should not be synced.
Once a field group is synced from the parent theme a copy of that group is saved in that acf-json folder in the child theme if it exists. Once this happens the field group is forever disconnected from the parent theme.
The “private” setting in the json file should be added to prevent syncing of the parent theme groups you do not want to allow editing of. This is explained on this page https://www.advancedcustomfields.com/resources/synchronized-json/
If they have already been synced they will need to be deleted from the database for the child theme site.
