I've had a similar situation at a playcentre with DD (who's 19 months) and a boy who was about 4.

The little boy was quite possessive over a pedal car but DD took quite a fancy to it too and although she's only little she's VERY determined.

Anyhow, the boy was quite rough with her and at one stage started pulling her out of the car. I'd seen his mum doing his best with him and she'd already stepped in quite a lot when he was doing similar things with other children. So I didn't blame her - I actually felt sorry for her because he was a handful but the lioness in me wanted to protect my cub, so I told him quite forcefully, "that's not cool" and in best Paddington Bear style (for anyone who watches that), "gave him a very hard stare."

He seemed to get the message and did not bother DD again.

For me, it came down to whether the mum was trying to do the right thing or not. In this case, like I said, she really was doing her best. If she'd been ignoring the situation, she'd have got a piece of my mind. If kids are going to be too rough and ruin everyone else's time while you sit and have a coffee, then that's not on IMO.