Meaning: The children were laughing, and their laughter filled everyone's hearts with joy.
Choice of Words: Children are full of energy and happiness, and when they laugh, it's like music to the ears. "Laughed" is the past tense of "laugh." Joy is a feeling of great happiness, and when it fills your heart, you feel like you're on top of the world. "Filled" is the past tense of "fill."