When our daughter was born, we had a chat with the consular officials
in Toronto aboutdual citizenship while registering her birth. They
said she could keep both until she was 20 and then she would have 2
years to choose one and renounce the other. In practical terms, this
means she can be a dual citizen until she is 22. (Our hope is that
Japan changes their policy by this time.)

We have Japanese friends in Canada who have both. They simply did not
inform the Japanese government when they attained their Canadian
citizenship. They just make sure to use the same passport entering and
leaving Japan and hope that nobody asks questions. We know of one who
was questioned but she managed to talk her way out of further inquiry
but I'm not clear about how this was done. Baffled them with bullshit,
I suspect.