from 1860s through to 1970s, children were sent by orphanages and workhouses in the UK to be placed as servants or be adopted in Canada (and to Australia & New Zealand from 1920s-1970s)
these children became known as home children or child migrants,
more than 100,000 went to Canada, the exact number that were sent to Australia & New Zealand is not really known,
so if you have a child in your tree who you cannot find a death for but seems to have vanished then you could consider it a possibility