Surely our adult relationship with our children is formed in their childhood? Yes, of course I realise that every family can have an 'awkward ' child, whatever the problem may be, but overall the child is father to the man/woman.
I had kind loving caring parents, as did DH and visiting them regularly, whether when we were first married and they were in their early 50s or later when they needed our help was always because our relationship was based on love and affection and we wanted to visit them or have them visit us. They were simply just part of our family. The idea of any obligation was never voiced because it never occurred to any of us.
We have the same relationship with our children. Our DS and family live 200 miles away and we see them every six weeks or so and are in contact almost daily. the same with our daughter, who lives 100 miles away and has just organised a very enjoyable outing for us as a birthday present and who will becoming on holiday with us next week when we go to France. Our DDil has exactly the same relationship with her family.