It's not a suffix. Both of these are terms of endearment based on the word 'diddums'. People call a very young child 'diddums' if he/she is upset - "Don't cry, diddums." However, we also say it to mock an adult if we believe he/she is behaving like a child - "You broke your fingernail? Aw, diddums!" So Rowling has made up two new terms of endearment based on this word (you are right about number 2 being based on Dudley). Dudley's parents speak to him as if he is a much younger child because he is spoilt. No. 3 - 'huge' is an adjective. The sobs are both huge and pretend.