A parent waits on land while the children come back from the water, and what they bring him is a stick. That's the whole scene, told in three short lines. The humor is gentle: a stick isn't much of a present, and yet the children offer it as if it were treasure.
The speaker's feelings are never stated, but the word "present" suggests he takes the gift seriously and with affection. He doesn't laugh at it. The title, "Terra firma," meaning solid ground, adds a quiet twist. The children have just left the unsteady, playful world of water, and the stick is a small piece of the earth they've handed back to him.
The mood is warm, light, and a little amused. The poem seems to say that the things children find worth giving are often simple, and that a parent can find real tenderness in something so plain.