If you already customize the site using main / parent theme, it will be lost if you switch theme including child theme. For export/import the customizer option you could use this plugin: https://wordpress.org/plugins/customizer-export-import/
For widgets, please try this plugin: https://wordpress.org/plugins/widget-importer-exporter/
But, if you have change the code, you may need to copy it manually into child theme directory.
*IMPORTANT: Please create backup before doing the export/import process.