Need a “pg_dump” command to take the backup of the child tables along with its master table for PostgreSQL database
You can specify multiple tables to "pg_dump" by specifying multiple "-t" flags.

If you are hoping to specify only one "-t" flag and have pg_dump automatically include all the dependent tables, that is not a feature pg_dump has.  You will have to enumerate the tables yourself, either manually or by querying the system catalogs.  I can't give you the query, because your question is not well defined.  We don't know how you want to handle recursive dependents or cross-dependencies, for example.