Thanks for the answers,
The children in question were 10, 9, and 7 years old.
The mother was Sarah Crunkhorn, the child in particular was William Smith, born 1829 in London. Someone else's family tree shows him travelling on the Elizabeth, June 1836, with his sisters and mother.
I'm battling with aus. records. william did make a life in aust., but after having a daughter in London in 1859.
Thanks for reading,
lasswade