whooops, posted before finishing..
www.familysearch.org/en/wiki/South_Africa_Genealogy
Is a good starting point for finding out what resources are available online. I would also suggest looking at passenger lists departing from the UK to see if you can find them as these often have family groups travelling together, or give a final intended destination. You might find them travelling back to the UK again after settling in S Africa.
The name changing thing/spelling is hard and it also depends how the database is structured, how it treat O' names. Some ignore the ' and others don't. Using wildcards is a good way around this. Putting in alley will return any Malleys OR Omalleys OR O'Malleys (or Palleys, Talleys etc). You can put in more wildcards - so mall*y if you've come across a record listing them as Malloy, or something.
On the other point about the child - to me the obvious assumption was that the child had either a physical or learning disability. It was VERY common for children to be institutionalised from a very young age and the parents encouraged to move on and forget about them. You don't say what period you are looking at, usually records of children;s homes or hospitals are classed as medical records and locked for 100 years, so if you are looking pre-1922 you may be unlucky. Before 1926 there was no adoption law and arrangements were made privately, or through a charity. Records were not always kept. It is entirely possible that a child who was born as Jane Smith went into an institution, and was "adopted" as a toddler by Mr and Mrs Brown, who decided they preferred the name Susan. Susan Brown grows up, marries, has kids and dies and has no idea that she was originally Jane Smith. Or does know, but would prefer that nobody else knew.
You could dig around into the relatives which she was initially with, see if anyone else pops up who could be her at an older age. Or if you can identify an institution, see if records exist. I would usually suggest DNA but if you suspect the child died young without having children of their own, that's not going to help.