you can provide some bool flags or even status function, which will describe the child object' state.

simple example:
Qt Code:
  1. enum State
  2. {
  3. Idle = 0,
  4. Running,
  5. Error
  6. };
  7.  
  8. int currentState(); //returns current state from State enum
  9.  
  10. State _state; //variable for storing current state
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then you can check child' state or even block parent object waiting for state changes.
for example
Qt Code:
  1. ChildObject obj;
  2. obj.run();
  3.  
  4. while(ChildObject::Running == obj.currentState)
  5. qApp->processEvents();
  6.  
  7. ...
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and another Qt-styled solution is using signal-slot connection, which is of course the best one.