I wanted to post, as I feel like I'm probably overreacting and being silly but the other half of me is genuinely worried.
A child in my DCs class (year 4) is always so miserable looking. I just can't help but notice the huge contrast between them and the happiness of the other children. I know their mother and although perfectly pleasant to make small talk with, she comes across as quite a 'mean' mum. I have never heard her say anything nice about her child, she's not affectionate at all, her child just trudges along while she barks at them all the way through the gate. The child is prone to not speaking, and is getting in trouble at school for bad behaviour. Then the school speaks to the mother, but I have a feeling this just makes it worse for them at home.
We have had them over to our house to play a few times and they have been fine but prone to getting in sulks and not speaking if something upsets them.
They have made the comment 'I wish I could go to your house everyday after school' etc.
The child is always telling me nice things thet do with their dad, etc. She has two younger children from two different relationships that she seems to treat a lot better (maybe because they are only small).
Some might say that she's just stressed from being a single mother but I just think there is more to it than that. In 5 years, all I've heard her do is complain and moan about her child. She is always making digs about how not clever, how overweight.
I just have this feeling in the pit of my stomach that something isn't right. I am far from the perfect parent but I have never treated my DC like this.
I am so worried that if she behaves like this in public, then she is probably far worse behind closed doors. 
Sorry for rambling...just trying to get my thoughts out.