You are happy where you are, although a bit cramped.
You expect your adult children to move out in the course of the coming year, which will leave you, your husband and your father in a four bedroomed house.
After your children move out, you will be three adults, presumably occupying two bedrooms, which will leave two rooms free for when your children visit.
You don't mention either your age or that of your father, but with three adult children it seems reasonable to assume that you and your husband are approaching retirement age.
You have good neighbours and live in a quiet street, so quite honestly, unless there is no public transport and no shops nearby and you are looking forward to the time when you and your DH should not be driving any longer, there is no reason on earth to move, and good reasons for staying where you are.
Why burden yourselves with a mortgage now, and a house that will very soon become too big?
How likely is it that all your adult children will come to visit at the same time? Are there no B&Bs or other reasonable accomodation nearby if they should all decide to come at once?
Finally what kind of retirement are you planning? Do you really intend it to be focussed on your family? You have done your duty by them by all accounts, and however much you love them, they should not be the only thing occupying your now.