Either to them or to anyone else?
I think you can say to a child that their behaviour was wrong but not they are a bad / horrible / irritating / selfish etc person.
Someone said to me the other day that their teenage daughters legs have become fat because she eats too much and I also think this is wrong!
I've never been overweight but my mum used to criticise my body when I was a teenager and it did a lot of damage.