The Mother & Child Isles are a pair of islands next to each other—the larger Mother Isle to the north and the smaller Child Isle to the south. A giant wall of sheer rock surrounds the Mother Isle, so you need the “Ballad of Gales” warping song to enter.
Blue ChuChu
Defeat the Blue ChuChu on Child Isle for a Blue Chu Jelly.
Silver Rupee (via Treasure Chart #29)
Find and open Treasure Chart #29 (at Windfall Island [D2]) to pull a chest from the Great Sea near the Mother & Child Isles. The chest contains a Silver Rupee.
Traveling Merchant
After you begin the trading quest side quest at Windfall Island (D2), three Traveling Merchants appear on three islands of the Great Sea, one of them here on Child Isle.
Fire/Ice Arrows Upgrade (via Great Fairy #6)
When you learn the “Ballad of Gales” warping song from Cyclos, you can enter Mother Isle by warping directly inside. The first time, the Fairy Queen greets you and upgrades your Hero’s Bow with the power to shoot Fire and Ice Arrows.