Assuming you know nothing about the mum really but have just spoken to her in passing at a playgroup you both attend.
Would you think the child is just going through a phase where they think there mum is mean but really it's just normal mum stuff? Or would you think the mum must have done something (or not done) to make the child feel that way and be genuinely worried about it? Would you try and reassure the child their mum does care about them (even though you honestly have no idea)? Would it be wrong to try and stick your nose into this child's life over an offhand comment like that? But then even if she does have a mum who cares deeply for her... does she feel that? Do they need more support?