I have two test for Child incidents. Both based on the out of the box Quick Start Test Suites. In one I use insert record 2 time to create two incidents. Then I open the second one I created and set the parent incident field to the 1st one I created. In my second test I create one incident using insert record and from there I use the UI Action of create child to create a child incident.
Link to incident quick start test suite:
https://[your instance name].service-now.com/nav_to.do?uri=sys_atf_test_suite.do?sys_id=3c10c110731423002728660c4cf6a739
