I look after a friend's three children each Saturday whilst she does a course - 8 weeks, 14 months and 3 years. They are all pretty good so it's usually easy, but I have started to find that when we are in the park other parents will often expect me to watch their children on high objects / climbing up objects with out asking - more of an expectation.
For instance, yesterday, I had the baby in the sling, the toddler was happily playing under the climbing frame and the three year old was up and over the climbing frame, wanting a hand jumping off, running back up to climb again. There was this little boy who as about 15 - 18 months and he was way too young but still trying to go up the climbing frame - the mother just say watching him / me keep lifting him off it because I physically couldn't watch him failing at climbing it and keep an eye on the three year old who definitely was too high to "jump" without being caught.
WDYD in these situations? We ended up going to feed the ducks which wasn't ideal but I felt I couldn't be responsible for someone else's child!