In the 1851 census there were two John Costains who could have been the one who went to Canada, or the one who was buried aged 2 Arbory 21 Mar 1853.

There was the one with his Duke grandparents, and the one in Arbory aged 3 with parents Thomas (32) and Ellen (24).
In 1861 (Brian's index) the Arbory one was aged 13 with his parents Thomas (42) and Ellen (35), plus other children. So not the one buried 1853.

So we need more information from Penny. What sources give the Canada one the Duke grandparents?
It looks possible that it was the other John who emigrated.

Sue