I can’t speak to the ducting of a heating system but I relay my thoughts on having the joys of grandchildren contact.
For me, mental health is a priority. Having contact with friends and family is important for my mental state. In regards to grand children, though they are out and about a lot and can be little germ bags (or vectors of disease), I feel they can usually be in proximity to me. The rules were and are simple, come visit with clean clothes, insure you are not sick, wash your hands when you get here. My wife and I have a very strong relationship, however we both need to interact with others to keep spirits up and those children are a great way to insure that happens.
Hope this helps.