Oh, i am sorry to hear that. If your children got a black eyes, you can adopt cold compress. Just apply a small bag of ice and put it on his closed eyes, it can help to reduce the back eyes. But it is not cure it. If his black eyes are not serious, the black eyes will disappear themselves several days. If it is serious, you'd better take him for an eye check by eye doctor and take some medicine.
In your situation, you can apply an ice pack for 15 minute every time. Then do it several times a day during the first 24 to 48 hours. And you should wrap the ice pack in a washcloth or paper towel and gently press against the area surrounding the eye. You should know that ice is the best way of treating a black eye and it will also help to numb any of the initial pain. You could also use a bag of frozen peas if you don't have any ice handy. But when you do all these things, you also should know that there may be some infection which can occur, just pay more attention to it.