hell no. Not unless the child was majorly over weight, and even then, i still wouldnt use the word diet! As the parent, and adult, then its your reponsibilty to ensure the child gets good nutritional food to eat, and plenty of exercise.
My SIL's DH has 2 girls, which they have full custody of, the girls are about 7 & 9. The 7 year old they call tubby, and by no means would i ever consider her to be fat, maybe she got abit of a puppy fat tum, but, i would never have noticed it myself, and even when it was pointed out, still didnt think it was anything. The year old likes to pull up her top, and show her little sister her ribs. And this practise is sorta encouraged.
All i can see is a 7 year old putting herself on a diet, so she can be more like her sister. Am concerned about it really.
Also, more concerned about my kids - i come from a 'not fat' (although some are) but, big boned, family, on both my parents side. DH's family are all slim. If my girls (and boys) take after my side, are they going to get the same sorta response from my DH's family, as they will be different from the rest of them, when, they are just kids, and more importantly, will be healthy ones at that.