Grindelwald figures Credence is too old and damaged to be useful, but his genes are clearly strong and hey, Graves' genes are pretty awesome too with wandless magic and whatnot.
So he decides to make them do it and train their offspring as perfectly loyal soldiers.
(Even if he doesn't succeed, and they do get rescued, Credence is heavy with Graves' child and he doesn't believe in abortion. And Graves isn't a quitter no matter how much he resents the way the child came to be).