I have got a similar use-case posted and luckily the answer to your question was steps to reproduce in one of the posts (That's the beauty of sharing the knowledge, you never know what could help)
Use-case:- Configure an onboarding case to create a benefits enrollment case as a child case. This requires the user to complete the benefits enrollment case before they can complete the onboarding case.
- Under the Benefits enrollment stage, click + Add process.
- Under the Benefits enrollment step, click + Add step. A pop-up opens to select the type of step to add.
- Select More > Utilities > Create Case(s).
- Click Select. A second step is added to the process.
- In the highlighted field, enter Select Benefits.
- In the properties panel for the Select Benefits step, select Create a child case.
- From the Case type drop-down field, select Benefits Enrollment.