I would hesitate to guess at the total number. Statistics Canada in 2006 came up with the figure of just over 70,000 Canadians who have declared that they work abroad. Those are people who have declared that they are working abroad. That was in 2006.
Given the mobile nature of this community, the 70,000 Canadians who were abroad then are likely not the same 70,000 Canadians who are abroad now, nor will it be the same 70,000 Canadians who are abroad in five years. To think that not a significant number of those 70,000 will have children would be naive.