Sub-Query can be used to Parent and Child records in Salesforce SOQL using REST API.
Example:
/services/data/v57.0/query/?q=SELECT+Name+,+(+SELECT+Name+FROM+Contacts+)+FROM+Account+WHERE+Id+=+'0013t00002XXRyDAAX'Workbench:
Postman:
Parent and Child records in Salesforce SOQL using REST API