Was at the playground today with DH and 2yo DC, one of us was following him carefully around the climbing frames etc.
2 other mums come into playground, with 1 DC each a bit older than mine, i'd say 4 yo. One mum had a baby in pushchair too. They were chatting and not staying very close to their DC who were running around playing together. First mum then took her DD to the toilet.
Second mum stayed in playground with baby in pushchair and DS playing. Her DS climbs on a big climbing frame and she watches him but doesn't stay close. She walks round the edge pushing her baby in pushchair and he goes the other way on the climbing frame. Next thing he falls off, the mum calls him but didn't see him fall off
He starts crying and she calls him again, leaves pushchair but still can't see him so my DH has to tell her that he fell off and where he is! 
To be fair to her she does go straight round to him and picks him up but he fell about 3feet! AIBU to think that she should have been watching him closer?? DH and i always supervise our DS 100% on play equipment and follow him around.