Thanks for all the answers, to clarify my concern with landing at Detroit is that it adds one extra step - acceptance for entry to US, on top of acceptance for entry to Canada. We would get visas for US (certainly for me, presumably for wife, kids, FIL also), but my worry was that they may somehow decide not to let one or all of us into the US (maybe because although I was planning to enter Canada immediately they would have concerns that I may get turned away at Us-Canada border and so would not let me into US?).
I don't think it should be a problem as we should all be fine to enter US as a visitor (once we have visas ofc) and if we are turned away by Canada we would be able to go back to UK without any question ( we all have British passports).
I guess I was hoping someone would either say "this is routine, US would definitely not turn you away if you were just transiting to Canada and have British passport anyway" or "US may well turn you away for X reason so it is not worth the risk". In the absence of a definitive answer then I have to weigh the risk of an extra border crossing (UK-US) against the hassle of getting from Toronto-Windsor. We will be 6 travelling including 3 children and one 92 year old so being turned away (or even detained) at the border is not a trivial issue for us.
@taffy7: we can rent a car at airport, but not one big enough for 6 people, 6 large suitcases, 6 small suitcases and a small dog afaics. We have looked at a taxi service but it is expensive and the best prospect would not take the dog
It looks like maybe we will get a family member to bring a 12 seater to Toronto, so we *may* be ok, but not sure yet so Detroit is still a possibility.