Six days ago, I sent some bitcoin and it hasn't been confirmed. In an attempt to execute a Child-pays-for-parent transaction, I used my mnemonic seed to derive addresses from my blockchain wallet(in order to access my private keys), but I can't seem to find the change address on the list of derived addresses. Where can I find it?
Go to https://iancoleman.io/bip39/ and download the page to use it offline (you should never enter private keys into a website).
Scroll down and choose the BIP 44 tab. In the text box next to "External / Internal", change the 0 to 1. You should then get all of your change addresses derived below.
The best way to get the change address is to check the transaction corresponding to the "known" address from your wallet from where the transaction was originated. Blockchain does not keep a track of the addresses it generates for change BTC for the wallet users. Also the blockchain wallet is BIP44 compatible which is basically a more standardized nomenclature for BIP32 and BIP39. Following that format you would be able to derive the private keys.
A change address can be any address. The change-address represents the left-over output of an input of a transaction. When splitting a UTXO of a large amount or using many UTXOs in one transaction, the total input minus the amount you are trying to transact represents your change amount which is directed to your change-address (the change address is usually just your receive address).