Ethics and Probability

Someone suggests you play Russian roulette on a child: if the bullet comes out of the gun (1/6) the child dies, if it doesn’t come out (5/6) the child receives a gift that makes him very happy. Is it ethical to play?
I guess not. But what is the difference between this case and this case where 2 people want to have a child, and there is a certain chance that the child will be sick and will suffer greatly in its life?