Mother and Child Isles

In, the Mother & Child Isles are a large island (the Mother) and small island (the child) situated next to each other in the Great Sea.

Features
The Mother island is actually a Fairy Fountain which serves as the home to the Fairy Queen. However, it is surrounded with a high rock wall. The only way to access it is using the Ballad of Gales, learned from Cyclos, to warp inside of it. Once Link rescues his sister Aryll from the Forsaken Fortress, he can play the Ballad of Gales to enter the Mother island and receive the Fire and Ice Arrows from the Fairy Queen.

On the Child island, which can be easily accessed by boat, Link meets a Traveling Merchant. It is also home to a Blue ChuChu, and Rown, one of the Korok inhabitants of Forest Haven, can be found here.

Mother-and-Child Rocks
The Mother-and-Child Rocks are a notable part of Zora's Domain in. The structures can be found in the central waters near the base of the waterfall, and are significant since it is the only area where the native Reekfish congregate. In the game, Link must catch a Reekfish in order to obtain the Reekfish Scent and thus find his way to Snowpeak Top.

The Mother-and-Child Rocks are an obvious reference to the Mother & Child Isles found on the Great Sea in The Wind Waker; however, it is unlikely that the two are related, as the rocks are much smaller in size than the islands, and because it's confirmed that Twilight Princess and The Wind Waker take place at the same time in alternate timelines.