Head to the Potion Shop inside the Skyloft Bazaar. Speak with Bertie, who does the whole Potion Infusion thing. He’ll speak of being too tired because of the baby he carries around (bet he knows how Yoshi felt in Yoshi’s Island).
Anyhow, sleep until nighttime and cross the river to the east side. Directly across from the bridge is Bertie’s house.
Go inside and you’ll learn that the baby’s rattle was stolen by a bird. Agree to help and leave the house. There are two ways to get the Rattle for the baby.
Easy Method: Stand next to the windmill where the Rattle is – it is quite close to the waterfall. During the daytime, wait for Beedle’s Airshop to float by. When it is close, stop it with by shooting the bell.
Ride the rope to the top. Get to where you can view the bird nest. Blow on it with the Gust Bellows to reveal the Rattle, then grab it with the Hook Beetle.
Somewhat-Difficult Method: Go to the waterfall and climb to the top of it with the Clawshots. Atop the waterfall, go to the other side with the Gossip Stone and look down. Line up with the nest, then dash-jump over the edge, tilting the Wii Remote/Joy-Con/right stick to get to the nest. Blow on it with the Gust Bellows to reveal the Rattle.
Either way, when you have the Rattle, go to Bertie’s house at night and hand the Rattle to him.