Target child of child without knowing name?

A simple solution is to create a ID script that basically contains the object ID and attach it to all children - like this:

 using UnityEngine;
 using System.Collections;
 
 public class ObjectId : MonoBehaviour {
     public string myId = "";
 }

Add this script to every child you want to identify, and set their IDs in the Inspector. You can then get all scripts of this type with GetComponentsInChildren and search for the one you want with a function like this (in a script attached to the parent):

 // return the transform of the desired object, or null:

 Transform FindById(string targetId){
     // get all ObjectId script down the hierarchy:
     Component[] children = GetComponentsInChildren<ObjectId>();
     // search for the desired ID:
     foreach (Component script in children){ 
         if ((script as ObjectId).myId == targetId){
             return script.transform; // if found, return its Transform
         }
     }
     return null; // if not found, return null
 }

EDITED: The old and good FPS Tutorial handled this by keeping only one of the weapons active at any moment and calling its functions by name via BroadcastMessage (SendMessage to any children). In order to shoot, for instance, it called the function Fire, which could be totally different in different weapons:

 if (Input.GetButtonDown("Fire1")){
   BroadcastMessage("Fire");
 }

The same approach may be used to get info about the weapon: BroadcastMessage accepts passing one argument to the target function, which may be a class reference - you can assign the desired info to the variables in such class, like this:

 using UnityEngine;
 using System.Collections;
 
 public class WeaponManager : MonoBehaviour {
     public class WeaponInfo {
         public string name;
         public string ammoType;
     }

     WeaponInfo info = new WeaponInfo();

     // call this function to set info fields with the current weapon info:
     void GetWeaponInfo(){
         BroadcastMessage("GetInfo", info);
     }
 }

And add the function GetInfo to all weapon scripts:

 void GetInfo(WeaponInfo info){
     info.name = "M9";
     info.name = "9 mm";
 }

When you call GetWeaponInfo(), the fields of the variable info are set to the values defined in the current weapon.

WARNING: I usually write in JS, thus the C# code above may have some stupid errors!