Larkin thinks that many people have children to be 'added to' as he puts it but Larkin believes that having children would decrease his power or 'dilute' him. This could not only mean decreasing his power but also decreasing the amount of time for himself as Larkin seems to believe that all your time is spent with your children once you have had them. Larkin also comments on that fact that he has not conformed to what society expects of you; 'to have no son, no wife, no house or land still seemed quite natural' this comment implies that Larkin is quite happy not having these things and that they were not for him.
Larkin also seems to think that the decisions you make shape your life and once you have made them you are unable to go back. 'Those warp tight-shut, like doors' the doors are metaphors for the doors and once you have made that decision, the door is shut forever leaving you unable to backwards. Therefore maybe Larkin believes that children are a mistake.
However, once again like in 'The Whitsun Weddings' Larkin appears to be able to see the positive side to having children 'embodying for Dockery and son, for me nothing'. Here Larkin seems to be able to see that some people enjoy the experience of having children but for him it would not have the same affect.