Previously, for a brief period, when the age of a child account reached a certain number of years (e.g., 13 in North America), it had to be unlinked from its originating parent account to take advantage of functionality in some games that use Pokémon Trainer Club, such as the ability to trade Pokémon in Pokémon GO.
This unlinking process is no longer required and there is no longer an unlinking button in the parent’s account settings. When an account reaches the appropriate age, that age will be reflected in the account settings and players will gain game functionality by logging out and then logging back into the game.
A child account will always appear as a “child” account under the originating parent account. This is only to provide historical information about account creation.