Version 1 – Although Peter exchanged regular letters with Joan and the children he never saw his family again as he was killed on active service off the coast of Norway in April 1940. Joan and the children returned to England shortly after VE Day in 1945 but they were never happy in their former home in Southampton and they all returned to Canada within a matter of months.
(This would be a book mainly set in war-time Canada and would be a fairly major research project if the mass of detailed information needed in the book was to be factually accurate.)
Version2 – Peter survived the war and Joan and the children were reunited with him in 1945. Sadly he and Joan, like so many married couples separated by the war, had drifted apart during the six years that they lived their own lives and they divorced in 1946 with Joan taking the children back to Canada the same year.
(The contrast between the sadness of gradually increasing emotional separation and the happiness of the war moving to a successful conclusion might have worked rather well)Version 3 – Peter survived the war and Joan and the children were reunited with him in 1945. Peter went back to being a family solicitor - based in Southampton as before. The children found post-war austerity Britain very different from the wealthier and more relaxed Canadian lifestyle they had got to know so well during their six years away and Peter never managed to establish a normal parental relationship with them. He was found drowned in the Solent in 1951 and an open verdict was recorded.
(This would be my first choice. Lots of potential for tension and an ending that would leave questions unanswered.)
Version 4 – Joan and the children never made it to Canada. Reports of intense submarine activity caused the liner to return to Brest only 48 hours after it had departed. Joan found Peter a matter of minutes before he was due to sign the papers committing him to naval service for the duration of the war. After much discussion the whole family sailed back to England and Peter saw his family safely established in rural Shropshire before he returned to Portsmouth to enlist. He survived the war and was reunited for good with his family in late 1945.
(This could be a "two threads running side-by-side" book - I think it could be quite effective and since there is lots of source material around it would be tolerably easy to research the fine detail required throughout.)