How to create an aspect on an Interface Method that extends from A "Super" Interface

Try this pointcut instead.

within(com.xyz.someapp.ChildServiceInterface+) && execution(* save(..))

The + indicates a subtype pattern.

Or you can put pointcut on all the methods of that class using

@Pointcut("execution(* com.xyz.someapp.ChildServiceInterface.*(..))")  
public void childServiceSavePointCut();

The * indicates all method type.