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 ).
If the behaviour is in any way dangerous to myself or any of my children, then I immediately step in and intervene, no questions asked. If the ehaviour isn't dangerous but is merely distressing for my child, I will usually look around to see if the other parent notices first, and also allow my child a chance to deal with the situation themselves - however I am ready to step in if it escalates or if my child has done all they can and the bad behaviour is persisting (will be different for different aged children, younger ones will need you to step in sooner). I don't step in for every little thing, and think it is good for kids to learn to handle things on their own - but I do definitely keep an eye on the situation and stay ready to step in if needed.
But I generally have no problem stepping in and telling any child "no", particularly if their parents fail to care (there is a big difference between not noticing that your child is hurting another, and noticing but failing to get involved). Generally most parents around seem to be OK with this. I am certainly happy for strangers to tell my children "no" if they are behaving badly. However occaisionally you will get some parent who thinks that their child is always above reproach. Tough luck to them, I feel sorry for those children.
When my DD was about 18 months old, she was playing in one of those car rides that you put the money in (we never do, it was just stationary). An older child (would have been about 3yrs) came up and tried to move her over to get in himself. The mother was watching and couldn't have cared less - just kept catching on her phone. I went over and asked him to be careful as she is only little. When I turned to look at the mum again, he 'pushed' her out of the car onto the floor. The mother was atching him, still didn't care. I was livid. I picked up my crying DD, Told the little boy " NO, that was very mean. You DON'T push little kids." (In my stern teacher voice) The mother suddenly got interested - offended at how dare I talk to 'her' little angel like that! She doesn't believe in disciplining children! That poor little boy is going to get into so much trouble in his life, and he is going to be clueless as to why, because his mum is failing to teach him basic respect for others.
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