My DS1 who has just turned five has started to notice that people are different, noticing that people are different colours etc. A few days ago we were leaving nursery when one of the other mums (a rather large lady) was rushing down towards us to get her little girl. My DS1 said rather loudly when she was hardly past us "Mummy that lady's really fat!", I hoped she hadn't heard. I took the opportunity to explain to my DS1 that we were all different shapes and sizes and perhaps rather stupidly thought it was a good opportunity to explain about healthy eating as they have been looking at healthy eating in nursery. I explained that one of the reasons we try to eat proper food was to keep us healthy and stop us getting too fat (made it clear that it was OK to have chocolate, crisps and sweets as treats but if we eat them all the time we become unhealthy and eventually fat), I also explained that it was important to be active, running about, swimming etc. (Seemed a good idea to explain all this at the time).
Today however we were at nursery early and so was this lady and her DD. We were sitting waiting and my DS1 stood up, went over to the lady and took his finger and plunged it into her stomach and announced "You're fat, you've got a big fat belly, you've eaten too much chocolate" I was mortified, apologised profusely to the lady who just said "It's OK, kids are honest aren't they" and was then rather quiet.
I'm not twiggy myself (see the Big Mamas thread) and I really felt for the woman, I feel awful.