Only social responsibility for me. My parents are relatively young and healthy and desperately missing their grandchildren. My mum is regularly in tears (that isn't normal for her). Therefore to me it's common sense we should take the (as I'm sure would be judged by those in power) very small risk and mix our two households.
(not mentioned the in laws, not because they are less important but they are slightly older, one just over 70, one just under, and, while they miss their gc, seem to be coping slightly better)