Use group.onChildAdded.add(function (child, parent){ // {child} was added to {parent}}); group.onChildRemoved.add(function (child, prevParent){ // {child} was removed from {prevParent}}); group.onChildKilled.add(function (child, parent){ // {child} was killed}); group.onChildRevived.add(function (child, parent){ // {child} was revived});