I have a feeling this may be more to do with cash flow than morality! I do not have the details to hand but in the mid 19th century ships became more controlled and registered as to passengers. An adult passenger became a "statutory unit" but two children under fourteen counted as one adult - i.e. each only half a statutory unit so they could share a berth = only half a fare. If she claimed the boy was only 11 he could have shared a berth with another child, not necessarily a relative, but they would only have to pay half the adult fare which would have been charged if his real age was known.