I think it's good you respond - suffering in silence is not healthy for yourself, good for you tackling the subject.

It sounds sensible that you further discuss with the children, yes they can 'say it like it is' I agree but when you discuss you are opening a dialogue and hopefully they will come to learn not to say hurtful things about appearances.

I am sorry your grandma said such things. You write in the past tense, I wonder if that means this no longer happens, sorry I realise you may no longer have your grandma. We need to feel accepted by our families, however we look, this criticism must have been hard to live with.

I think it's good you are recognising that people say these things because it is their behaviour and values and that it does not reflect you- you sound like you have a good sense of self identity and what is important to you Good for you!