You would expect grandchildren with a changed name to be the children of the daughter of the grandparents. Since the grandparents are named Christian and their only daughter of the right age still living is Margaret try looking at Thomas Christian and John Christian listed only to Margaret. She is also living with her parents in 1851.
It may be she simply adopted the name of the father and you will be struggling to find it.
IGI states John Christian died 1863.

Just a thought

Len Faragher