I want to tell my GFs kids to "dont grow up to be like her"
That sounds strange why should they not want to be like their mother?
I think she has poor quality of character.
I get that people mess up sometimes and do some undesirable things. I still do not think that you should tell her kids something like that though.
I'm sure my girlfriend fucked up with me through and through
You should not bad mouth kids parents to them. They need to be able to trust their parents and telling them bad thing will only hurt them.