A use case does not have a child sequence diagram.

A use case describes a piece of system functionality, sequence diagrams describe how they will be implemented.

Without a sequence diagram it may be difficult to infer how functionality should be implemented.

Trivial use cases may not require a sequence diagram (although there is still an argument for providing a trivial sequence diagram) and these can be ignored.

If the sequence diagram exists but is not linked to the use case then it can be linked using the child diagrams dialog.

If the functionality has not been designed: right click on the use case, select New > Sequence Diagram, and add the design.