Other children born to this couple?
6978/1862 AGNES M CASSILIS
2956/1864 ANNIE GLEBE
3074/1867 EDWIN J GLEBE
3464/1869 ALFRED J GLEBE
9620/1860 EMMA MUSWELLBROOK
It is possible that the two children you mention were not those of John C SHUGG, but from a liaison between Elizabeth and another Man

The other man, possibly FIRTH? Could be aboriginal?
Neil