Usage: keychain.getKeyForChild(index,type)
Description: Use to derivate the root key to a specific child. (useful when Wallet is initialized from a HDPublicKey)

Parameters:

parameters	type	required	Description
index	number	yes	Enter a valid index
type	string	no (default:HDPublicKey)	Enter a valid type (one of: ['HDPrivateKey','HDPublicKey'])

Returns : HDPrivateKey

Example:

const { privateKey } = keychain.getKeyForChild(0);