What is the recommended way to upload teenage volunteer and their parents data from csv file
You can use relationships to tie the parent(s) to the teen.

https://docs.civicrm.org/user/en/latest/organising-your-data/relationships/

Here is some information on importing relational data on this page. Just scroll down about 3/4 of the way down:

https://docs.civicrm.org/user/en/latest/common-workflows/importing-data-into-civicrm/