I say, and with God's success: His ablution is not invalidated; because it is a condition for the praying person whose ablution is invalidated by laughter to be of age. If a child laughs, his ablution is not invalidated; because it is not a transgression against him. Refer to: Sharh al-Wiqayah, 2: 33-34, and Tabyeen al-Haqaiq, 1/ 11, and Fath Bab al-Inayah, 1/ 68, and al-Ikhtiyar, 1/16-17, and Mukhtalif al-Riwayah, p.344-345, and God knows best.