What if these people were made sterile via radiation? Maybe a nuclear ending to ww2 did still happen. Another idea (and more likely IMHO) is that they are kept from having kids because they are starving. Resources are scarse,and everything is being used to keep the public from panicking (via these joy pills). Maybe there are elements of the supernatural at play here,perhaps the British pledged their children,and any yet born children to cthulu (or some other entity) in a last desperate bid to rid the Germans from their nation. If you think about it,rationing would be in effect,people would be subjected to Nazi concentration camps,systematic book burnings,and extreme cruelty to crush any resistance that did arise. Knowing the fate that their island will suffer,and the rest of the world,they make this sacrifice. In an effort to forget they create joy,and find ways to continue to survive;as people tend to do.It would be extra interesting if they had made this pledge to cthulu not necessarily meaning it,wanting it to happen but not understanding the results.TLDR sorry for the wall of text,basically sacrificing children (super sick I know) in order to avoid Nazi occupation of Britain via a pact with cthulu.