Simple answer: On the iancoleman site, under the heading "Derivation Path", change "External / Internal" from 0 to 1. This will show your change addresses under "Derived Addresses".
Slightly longer answer: When you create a bitcoin wallet, the default derivation path for the first address is m/44'/0'/0'/0/0.
BIP44 explains what these numbers mean.
The first number is purpose. This is 44 for P2PKH addresses, 49 for P2SH addresses, and 84 for P2WPKH addresses.
The second number is coin type. This is always 0 for bitcoin.
The third number is account. This defaults to 0, but changing this number allows the same seed to generate an entirely different and separate set of addresses.
The fourth number is change. 0 is for external chain (addresses other people will send to) and 1 is for internal chain (addresses which you will send to - i.e. change addresses)
The fifth number is index. Each address is numbered sequentially.
As you can see on the iancoleman site, the addresses you generate will all be m/44'/0'/0'/0/x, with x increasing by 1 with every new address. This is listing all your external addresses. If you change "External / Internal" to 1, it will start generating addresses with m/44'/0'/0'/1/x, which are your change addresses as described above.
