Words that I don´t understand
I am reading a Harry Potter book and there are some words that I don´t understand.

1 - Sweetums
2 - Duddydums (I believe this comes from his name "Dudley"
3 - To yell between huge, pretend sobs (Huge is an adj., but in this case I don´t know. I think that here it is a name).

PS: Is "(d)ums" a suffix? I´ve never seen this in a word but these ones.

Thank you in advance

Question

Words that I don´t understand
I am reading a Harry Potter book and there are some words that I don´t understand. 

1 - Sweetums
2 - Duddydums (I believe this comes from his name "Dudley"
3 - To yell between huge, pretend sobs (Huge is an adj., but in this case I don´t know. I think that here it is a name). 

PS: Is "(d)ums" a suffix? I´ve never seen this in a word but these ones. 

Thank you in advance

Answer

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.