The mood is the emotional landscape of the story, or how it makes the reader feel. In the first sentence, the mood is very happy and peaceful, but in the second paragraph we get hints that something is wrong and the mood switches to apprehensive.
The first paragraph establishes a wonderful summer day, using words like “clear and sunny,” “fresh warmth,” flowers “blossoming profusely” and “richly green” grass. The reader gets a very pleasant feeling, and the mood is pleasant because of these pleasant associations. Most people associate warmth, summer, and flowers as positive things.
In the second paragraph, the mood switches. The children are described as freedom sitting “uneasily” with them. They sit quietly together, and talk about school. While the girls stand talking amongst themselves, the boys begin to gather stones. We know that there is a reason for the gathering of the kids and picking up stones, and we begin to wonder what it is.
Bobby Martin had already stuffed his pockets full of stones, and the other boys soon followed his example, selecting the smoothest and roundest stones.
The boys guard their pile from “raids of the other boys,” which is another sign that things are not right. At this point, the mood is not peaceful and happy any more. The mood is anticipation and dread, and unease. We have a sense of foreboding, and we know that something is about to happen.