I don't mind kids, in some cases I even like them, but I shouldn't be expected to entertain a random child just because I am, what? The only female on site? Someone who happened to give your child some attention?
Parents should be more worried if their child has no issue with not only speaking to a stranger but following them around and even getting into their car (the child was not invited, we were having a tea break, sitting on the comfy seats, and he jumped in and kept touching my car). Sometimes you just get lucky and the stranger isn't a freak and has no interest in taking your child, but there could be that one guy who just drives off and you never see you child ever again. Just saying.
I was very thankful when the father and child drove off and we were able to finish without babysitting. It is very simple, never assume that your child will be accepted on site or that there will always be someone there to look out for them. Please, until your child is old enough to understand that vehicles are dangerous things to play around and that other people have to get their jobs done, find alternative care; the mother of the child, day care, grandparents.
Your child is not my responsibility. I didn't forget to use a condom, you did, so take care of the resulting kid. Oh, and don't forget to thank the person providing impromptu child care for free, jackass.