Discuss this case in tutorial. If you think it's wrong of the second mother not to have given birth to another child instead of the first, that slights the interests of the existing child. This case illustrates the role of imaginative thought-experiments in the moral life. For a self-conscious being, the birth of another being, however happy, is not great compensation. Creating new preferences isn't necessarily an absolute good.
Satisfaction merely compensates for the pain of desire.
An itch is painful, scatching merely cancels that pain. The itch and scratch model. Then there's plenty of room for positive value in new lives. The more you have invested, the more you have to lose. At the beginning of life, all possibilities are wholly abstract.