I am part of a "formal" playgroup too with a 22 month old.

My DD is a bit precious about toys so when another kid grabs the toys she is playing with I reinforce with MY child "Its ok to share with X why don't you play with this one here" EVen though the behaviour that caused the upset was from someone elses child I think its more appropriate in that instance to reinforce the "sharing" aspect with my own child.

DD is also very very precious about HER slide (the one at playgroup) and gets very grumpy if other kids want to play on it. I discourage her grabbing and pushing away other kids and again reinforce the "sharing" aspect and say "Its X's turn you can have a go next"

Throwing of stones or toys or anything then I would step in regardless if the mother wasn't around or was and another child was at risk of being injured. I usually say "Don't throw XYZ, its not nice and someone could get hurt". If another childs parent allows rough play thats fine for when that child is in their own home, but when they take that behaviour into settings where the child could hurt someone elses then I have no issue with stepping in.

If DD was ever to be like that with someone elses child I would be fine with that parent saying something to DD .... mind you I am usually watching her as I know she gets stroppy with what she thinks is hers

On a different line we live with my SIL and her DD behind us and little Niece (B) is a really rough kid always has been. They think its a little bit funny and while they discourage too much rough play they have the attitude that "kids need to work it out for themselves" Which is not how we feel about kid rearing. Yes they need to sort some things out for themselves when they are capable of knowing what is acceptable behaviour and whats not. They are both under 2 and do not have that level of understanding. I try to distract B ifshe gets too rough and upsets DD but if I see her hitting or biting then I get right down at her level, point my "No" finger at her and say in a stern voice "You do not bite/hit" ........ I have had daggers thrown at me when I have stepped in on that behaviour but I will not tolerate that behaviour. I prefer DD not to play with B so I make sure they have limited contact and ALWAYS under supervision of myself or DH (manily because IL's smack for bad behaviour which we don't do ).

HTH somewhat

Nae x