This scene is showing the event of which Sergeant-Major Morris is giving Mr. White the magical Monkey's Paw. This contributes to the overall theme because he warns Mr. White to be careful about what he wishes for, because there may be some terrible consequences.
Despite the warning, Mr. White carelessly wishes for 200 pounds. This proves the theme because he has no thought process of what the paw may bring. He is not at all careful for what he wishes for.
I don't see the money and I don't think I ever shall!
Mr. White realizes he had made a terrible mistake. He is granted with the 200 pounds, but at the cost of his son Herbert's life. He then realizes he should have been more careful about what he wished for.
He is hurt, but in no pain.
Mr. and Mrs. White feel ashamed of their wish. They then realize that they have more wishes left and use one of them to bring their son back to life. Mrs. White however is still uncareful and quickly wishes with no caution.
I wish for my son alive again!
Unfortunately, they are too late. While mourning the loss of their son, they realize they should have been more careful about what they wished for, which contributes to the overall theme.