'Children' is not only the plural of 'child' - when you use 'children' as a group or collective noun, it is a singular thing.
You can be 'in' a group, but 'among' individuals. 'Among' recognises the individuals within a group. Think of this - you can be in a car. You can't be 'among' a car.
Your specific example should use 'in'. This is because it is saying that something causes a certain result in a particular group of the population, children.
You tend to see 'among' used in similar contexts when the group is not as strictly defined. For example, "there has been an outbreak of chicken pox among children". This is because the illness is not limited to that group, but it is recognising that the individuals involved have largely been children.