"Both of them" // "Both of whom"

I’m trying to understand and explain to my students why ‘both of whom’ is the correct answer in the following sentence:

"Credit must also be given to Katheryn and Aiden, both of whom I am immensely grateful for."

I understand why ‘both of whom’ would be used here, but I can’t seem to explain why ‘both of them’ wouldn’t work as well.