After surveying parents of overweight children researchers found that the majority are not even aware their child is overweight:

Researchers with Deakin's Centre for Physical Activity and Nutrition Research surveyed more than 1200 families to find out if parents had concerns about their children's weight and if they took any preventative action to avoid obesity in their children.

The study of more than 1100 families found that 89 per cent of parents of overweight 5--6 year-olds and 63 per cent of parents of overweight 10--12 year-olds were unaware their child was overweight. It also revealed that 71 per cent of parents of overweight 5--6 year-olds and 43 per cent of parents with overweight 10--12 year-olds did not think their child's weight was a problem.

"These are quite troubling results and suggest that current obesity prevention campaigns are not hitting the mark with parents," said head of the Centre, Professor David Crawford.

"Parents are part of the front line in the battle to reverse the trend of obesity in children, it is therefore essential that they are armed with information and practical strategies that they understand and can easily build into their daily lives."

Professor Crawford said it was not altogether surprising that many parents were unaware their child was overweight given that "many adults are not able to recognise overweight in themselves." (Emphasis mine.)

I think the last part is the biggest problem. If you are not able to recongize that you are overweight, you are never going to have a realistic assessment of your child's weight.

Furthermore, the best parenting strategy with children and healthy weight maintenance is to lead by example. If the parents aren't maintaining a healthy lifestyle, their children are just going to look at them as hypocrites when they ask them to be healthy. You can't watch 10 hours of TV a day yourself and expect your kids to not do the same thing.