How to build sub module's child module via maven build argument

Check if adding the -am or -amd option helps

mvn clean package -pl sub_module -am
mvn clean package -pl sub_module -amd  <== (confirmed by the OP)

See here:

-am, --also-make

If project list is specified, also build projects required by the list

-amd, --also-make-dependents

If project list is specified, also build projects that depend on projects on the list.