Gameobject into arrays and choose someone in a random way,

Putting a lot of gameobject kids into a gameobject will randomly turn one of them around me, how can I do that the selected gameobject will become active,

If by kids you mean there is a parent GameObject, and you want to activate one of its children randomly:

// I assume this script is on the parent GameObject.
transform.GetChild(Random.Range(0, transform.childCount)).gameObject.SetActive(true);