It's too late for a condom!!!
Joking aside, there are plexi-glass cover that you can put on flat screen tv to protect them, maybe there are some for computers too. But like others said, vesa mounting it on the wall will get the job done.
But the easiest way would be to tell them not to touch the iMac? I have 3 kids, 2 of them are 6 years old boys and they play all the time in my home office and there have never been any accident with my iMac and I bought it 3 years ago. Besides, the screen is pretty tough, unless they trow something very hard in it like a car toy or something, it is not going to break. How old are they?
My main concern would the possibility that the iMac would fall from the table if the kids move the table while playing around ---> vesa mount on the wall
If they trow something on the iMac, get back at them by spraying them with holy water, that will teach them.😱