Set child gameobject active?

How can you set the child of a gameobject active I have tried

gameObject.transform.child.gameObject.SetActive(true);

But I get the error

NullReferenceException: Object reference not set to an instance of an object
Boo.Lang.Runtime.RuntimeServices.GetDispatcher (System.Object target, System.String cacheKeyName, System.Type cacheKeyTypes, Boo.Lang.Runtime.DynamicDispatching.DispatcherFactory factory)
Boo.Lang.Runtime.RuntimeServices.GetDispatcher (System.Object target, System.Object args, System.String cacheKeyName, Boo.Lang.Runtime.DynamicDispatching.DispatcherFactory factory)
Boo.Lang.Runtime.RuntimeServices.GetProperty (System.Object target, System.String name)
UnityScript.Lang.UnityRuntimeServices.GetProperty (System.Object target, System.String name)
BuildZone.Update () (at Assets/my assets/Scripts/BuildZone.js:24)

the script was attached to the parent of the object I was trying to make active
How can I make the object active?

Transform has no child property, so this should not even compileā€¦

To access children of a Transform, please use:

for (var child : Transform in transform)
{
    if(....) // check if this is the child you want to activate, by name, tag or anything else
    {
        child.gameObject.SetActive(true);
    }
}

If you have one child only, then of course you can skip the if statement. You can also access a child by its index using GetChild method.