Thanks for starting this thread Jus, as you know I was thinking about this lately too.

In a way, it's nice to know there is that community feel to playgroup- parents feel comfortable pulling up kids on their behaviour, and you know that someone is always keeping an eye things. But if were me, and there was no immediate danger to my child (and as you said, your tiny dd is so match for a 3 yr old), and the other kids mum were standing right there, I would give the mum an oppurtunity to say something first before I stepped in, ykwim? And then i phrase it a bit more gently. And after that I'd probably throw the other mum a warm smile and make some remark about how funny and determined they are at that age to smooth things out.

Maybe this mum just isn't very tactful... I understand why it threw you a little, would have had the same effect on me.