A very important issue which is largely ignored in these situations is the feelings of the child and their relationship with their parents. Of course health is important and we all worry that our children are eating a healthy, balanced diet and will grow into healthy adults, but from personal experience this has to be handled sensitively.
I had a father who was always very fit, slim and saw things as black and white like some of you do i.e. 'You eat less and exercise more, you will lose weight'. For him there was no such thing as 'big boned' you were fat and that was that.
Coupled with being weighed at school and always falling in the slightly overweight category (despite being the county cross country champion, muscle does make you weigh more) because build, athleticism and sex were never considered, I developed serious emotional issues around food. Yes even at primary school age.
I have largely overcome this, but I have no relationship with my father to speak of.
Some of you sound very blunt and I hope you do not convey such blunt attitudes to weight, body image and health to your children. Believe me it can lead to a lot of heartache for a sensitive child, such as I was. It can also ruin your relationship with your child in the long term.
I have managed to create a very relaxed atmosphere about food in our house (I have a son 2.8). He loves healthy food but does indulge in the odd treat. He is active in a way in which he enjoys. He is never weighed, as I wish him to develop a confident self image that does not revolve around weight and numbers. I can tell by looking at him that he is not overweight.
I do not feel it is necessary to weigh children routinely and Iwould be delighted if my children never discovered scales.
If a child becomes obviously overweight, it is possible to change things without making weight an obvious issue. If we are just talking about a bit of puppy fat, this can drop off as the child grows. If we are talking about bad eating habbits that have developed because of parenting, this can also be changed without too much fuss.
Finally if the worst comes to the worst and your child does experience weight problems into adulthood, just love them, love them and then love them some more. Praise their attributes and sympathise with any problems. Do not let it ruin your relationship with them.