The major theme that we can use to link these two stories together is actually the way in which striving for happiness or for a better world can so often turn out to bring more sadness and pain into our lives than we had at first. Often, the solution makes the problem even more profound, and both of these short stories attempt to show that through the action that occurs.
In "Harrison Bergeron," it is absolutely clear that in their attempts to create a world in which nobody feels stupid or unskillful, the likes of Diana Moon Glampers haver actually created a nightmare dystopia that goes to absurd lengths to achieve true equality. This is shown through the various hindrances or restrictions that characters legally have to wear, such as the earpiece that distracts Harrison's father every few seconds because of his superior intelligence. Vonnegut has taken a good principle, equality, but then taken it to his extreme to such an extent that he has created a dystopia.
In the same way, in "The Fifth Child," the Lovatts seem to be the perfect Christian family. They have their four children, and seem to have attained the happiness that they have always dreamed of having. However, when they continue to have children, everything changes with the arrival of Ben. They are forced to accept that they have brought an anti-Christ figure into the world that destroys their happiness and brings darkness and depression into their lives. Implicitly, this story suggests that seeking happiness too much or too earnestly can actually ensure we receive the reverse.