var script : ScriptName;
script = gameObject.GetComponentInChildren(ScriptName);
script.DoSomething ();

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    public ScriptName script;
    void Example() {
        script = gameObject.GetComponentInChildren<ScriptName>();
        script.DoSomething();
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    public script as ScriptName

    def Example():
        script = gameObject.GetComponentInChildren[of ScriptName]()
        script.DoSomething()