William Henry born 1854 was the child of Mary Crossman and "Captain" Crossman.
The problem with this date is that Edmund Douglas Crossman was supposedly fighting in the Crimean War at this time and had not yet arrived in the Cape.
What other alternatives are there?
a) they are children of Mary Stuart from a previous marriage or perhaps no marriage at all. The suggestion that she may have had children out of wedlock might be inferred by the St Patrick's Catholic Church scribe who wrote upon her death "she led an irregular life". I'm not sure what that is supposed to mean.
b) they were adopted by Mary and Edmund. But why? What circumstances could lead to that?
c) William, Charles and Ada Alice are the children of neither Mary nor Edmund but entirely different parents, the mother being Mary and the father a Captain Crossman.
This seems unlikely however, because, William and his wife had 10 children and used the middle name "Douglas" for two of their daughters. That seems to be a reference to Edmund Douglas.