Your expectation for carers of 4 year old is what? 1 carer per child? Eyes on all children at all times?
Rooms and playgrounds may have areas 'out of view'. There are toileting areas that are not 100% in view from every angle because we don't put toilets in open areas.
There may be a fort/slide. You cannot view the entire space from every angle 100% of the time. Lets not have one of those. Too many areas where kids are not visible. The list goes on and on.
The carers are looking after children. Multiple children.
They may be helping a child that fell over and another may be as stuck and need help.
For that 30 seconds to a minute something happened.
I am not discounting that it is a horrible situation.
But continuously flaming the carers when you were not there, you have no idea what the childcare is like or what the area is like is not on.
Would you seriously say the same to a mother if this happened in the park and the mother did not see?