This happened on Friday but is still rankling so I thought you could all tell me IABU so I get some perspective...
I took DD (3) and DS (5m) to the park on Friday. DD rushed off and started playing on the climbing frame while I got DS out of the buggy. I then heard someone shouting, turned round and saw a woman telling DD off quite forcefully because she had been climbing up the climbing frame at the same time as this woman's toddler son. DD had climbed around as far as I could see - she hadn't trampled the toddler. Anyway, I got DD down, apologised to the other mother and told DD she had to wait to make sure littler children had finished climbing up. DD decided to leave it and go off and play on the swings instead.
A few minutes later, this mother came over to the swings with her toddler and older DS who looked about 5. All swings were in use. The 5yo announced he wanted to go on the swings and grabbed DD's swing and pushed her off. He then got on. The mother didn't say anything at all. I stopped the swing, told him that was not a nice thing to do and to get off the swing as DD was playing on it. He made a fuss but did so. His mother didn't say anything but gave me very evil looks.
I feel quite annoyed about it still. I have no problem with her telling DD off but think she should have told her own child off for something which (IMO at least) was worse.