Happy to share that Seibert Media (@Martin_Seibert) has sponsored the development of Table MultiSelect feature.
This was frequently requested by many community members as this is very intuitive and quick and results in better user experience.
Instead of a child table and ‘Add Row’ button, you can simply select multiple values in the same field.
This feature has been developed by @netchampfaris and is available in the latest develop version.
Link to PR: https://github.com/frappe/frappe/pull/6675
Documentation is available here https://erpnext.org/docs/user/manual/en/customize-erpnext/articles/table-multiselect
Below is the GIF showing Table MultiSelect field in action.