to have been sarcastic back towards this mother who said my.dd looked overweight.

[watchoutforthatsnail]

Her son is tiny. 6 years old and in age 4-5 clothes. She was complaining about how little he eats and How he needs to eat all the time.
She said I wouldn't know becase my dd is overweight.

Dd is not. She is the height, and consequently weight, of an 8-9 year old child.
The doctor is happy with her. She eats pretty much everything put in front of her, all home cooked meals.swims once a week. Rainbows. Dog walking everyday. In the park, bike riding. Scooters etc.

So, I commented it must be very hard to have a child that won't walk more than 200 meters. Or do any physical activity. ( he doesn't, there is nothing physically wrong with him)

Ffs, where do people get off?

I'm sure I've been rude before. But I've always known it was wrong immediately.

The absolute best rule of thumb is that it is never acceptable to be vile about a child. No matter the circumstances.

So what age is it ok to be vile about someone?

YANBU

Nipped it in the bud make it clear her family is the one with the problems.

Wish I'd done that with 'friend' and her DH. My DCs are normal weight, height. I know because they made me so paranoid that I've checked several times despite no prior concerns. Friend knew my parents have poor health due to obesity.

Her DC is very small and slight if they have concerns they could easily get it checked instead my DCs got to hear increasingly vile comments that have only stopped when I resorted to being very rude back. If I'd been rude or at least less non confrontational from start would have saved myself some angst and maybe they'd have got of their asses and got their DC checked out and dealt with any concerns they actually have.

Don't feel bad about it, it's incredibly hard to stay calm when someone is saying something horrid about your kid and given that the child in question didn't hear, what you said doesn't sound all that bad to me if not exactly perfect. No harm done. I know that protective indignant feeling of rage all too well, my DD had issues with her appearance (due to a medical condition) and it's incredible how thoughtless people can be, even in front of your child. Chalk it up to experience and get yourself a good stock of sarky non-kid related lines in your head ready for the next time in advance. Also, I'm sure you know this but I think you'd have been better to quote past examples of the individual's bad behaviour in your OP, context is everything!