How do you change back to a kid in ocarina of time?

Turn into an Adult

After getting the third Spiritual Stone, return to Hyrule Castle. Approaching the drawbridge will start an automatic sequence. When the sequence ends, dive into the water to get the Ocarina of Time. At this point, another sequence begins and Link will learn the Song of Time. Enter the Hyrule Castle, go into the Market and walk up the steps next to the Happy Mask Shop. Go to the left and enter the Temple of Time. Walk to the far end of the temple, stand on the red carpet next to the altar and play the Song of Time. Enter the new opening in the back wall, walk onto the platform in the middle of the room and Grab the Master SWord. After an automatic sequence, Link will be given the Light Medallion and become an adult.