Does Keychat do this in order to derive many children pub/priv key pairs?
Keychat implemented NIP 06: Basic key derivation from mnemonic seed phrase
https://github.com/nostr-protocol/nips/blob/master/06.md
So users just need to memorize 12 words to have a self-sovereign ID. 
Discussion
No. Writing 12 words is easier than writing private key.
12 words == only one pub/pri key pairs.
But Keychat allows users to generate many mnemonic seed phrase. So users can have many IDs.
Could you integrated / create a bip-85 style key derivation option? With that you could create a lot off derived keys for different "nym"IDs but only need to remember one mnemonic to rule them all.
Different mnemonics can provide better identity separation; for example, if a mnemonic is leaked, it will not lead to the exposure of all IDs.
