today it would be inconceivable to leave behind a child of three but remember that in early 19th C there would be an extended family - maybe the child was devoted to a grandparent or who had been 'adopted' by a childless brother/sister - or even if the child was ill (smallpox or whatever) at the time the family was due to sail - have you checked for her appearance in Manx census but connected with a family member ?