BMI is accurate for most people. It drives me round the BEND when people - usually overweight people - trot out the line about it being a useless measuring tool because it would put most athletes at overweight.
Look at the weights of professional athletes. When they're competing, apparently Victoria Pendleton weighs around 60kg, Jessica Ennis-Hill is 57kg. Both, therefore, around 9 1/2 stone. And they are about as muscular as you can imagine average-height women being.
Yes, there are exceptions when it comes to fifteen-stone male rugby players but these are few and far between, and the same logic cannot be applied to the general population.
I'm 5ft 6ins. My BMI says I am "healthy" anywhere between around 8 1/2 stone and 11stone. Which is about right. At 11 stone (and I've been there) I'm a chubby size 12/14. At the moment I'm 10 stone and a size 10. I know that different people carry weight differently, but a "healthy" BMI covers a broad range of weights.
If people are far above the top end of a healthy BMI, they're kidding themselves - and vanity sizing etc don't help.
Anyway, back to the OP. To be fair, OP, I think what you do is far more important than what you say. Make exercise an important part of your family's life and lead by example with healthy eating.
I hope that by regularly going running, cycling and walking together my DS will see exercise as something "normal", not something that people do as a chore or purely for weight loss.
In terms of what you say, I think it depends on the child as to how they take it. My mother was very much of the "fat-is-bad" club, but me and my sister grew up completely differently. I'm a size 10, she's a size 22. Both raised exactly the same.
The key difference between us was, as soon as she was able to help herself to food, my sister would eat and eat and eat. She also hates exercise and will get a taxi if even travelling less than a mile. Whereas exercise is a huge part of my life, and if there's biscuits in the house I don't have to eat them all at once.
I guess it comes back to the nature/nurture debate. You never know how your children are going to grow up, but if you instil healthy eating and exercise in them from a young age, then they've got that healthy grounding there.