BIP85 enables you create several independent BIP39 mnemonic seed words (private keys) for several Bitcoin wallets from a single, primary mnemonic seed words (private key).
All you will have to do is remember the primary mnemonic seed words AND the index numbers for each derived mnemonic seed words.
The derived mnemonic seed words are completely isolated. These derived seed words can therefore be used for testing wallet software, etc.
Great also for setting up younger ones, or spouse, or parents with a bitcoin wallet.
Blockstream's Child Recovery Phrase allows you do the following:
  1. Choose an index number based off your BIP39 mnemonic seed word (private keys)
  2. Choose mnemonic seed word length.
  3. Blockstream Jade would then show you your new derived recovery phrase (independent mnemonic seed words).