In Splunk Phantom, child playbooks can access the action results of a parent playbook through the use of the Scope parameter. When a parent playbook calls a child playbook, it can pass certain data along by setting the Scope parameter to include the desired action results. This parameter is configured within the playbook block that initiates the child playbook. By specifying the appropriate scope, the parent playbook effectively determines what data the child playbook will have access to, allowing for a more modular and organized flow of information between playbooks.