Collision detection with raycast

Right now I have a script that works fine to control my flying ai cube in 3D space. however i'm trying to get collision detection in for the next step. I'm trying to do this with ray casts and it detects other objects just fine but any child of the target thats between it and the center of the target is also being considered and obstacle and I don't want that. how can i get it to ignore the children of the target object but not everyother object? here's the script:

    var target : Transform; //the enemy's target
    var shot : Transform; //bullet to be fired
    var moveSpeed = 3; //move speed
    var rotationSpeed = 3; //speed of turning
    var attackThreshold = 1.5; // distance within which to attack
    var chaseThreshold = 10; // distance within which to start chasing
    var giveUpThreshold = 20; // distance beyond which AI gives up
    var attackRepeatTime : float =1; // delay between attacks when within range
    var lastFire=0;
    private var chasing = false;
    private var attackTime = 1;

    var myTransform : Transform; //current transform data of this enemy

    function Awake()
        myTransform = transform; //cache transform data for easy access/preformance 

    function Start()
         target = GameObject.FindWithTag("Player").transform; //target the player


    function Update () {

        // check distance to target every frame:
        var distance = (target.position - myTransform.position).magnitude;

    if (distance < chaseThreshold) {
                chasing = true;

        if (chasing) {

            //rotate to look at the player
            myTransform.rotation = Quaternion.Slerp(myTransform.rotation,
            Quaternion.LookRotation(target.position - myTransform.position), rotationSpeed*Time.deltaTime);

            //move towards the player
            myTransform.position += myTransform.forward * moveSpeed * Time.deltaTime;
            // give up, if too far away from target:
            if (distance > giveUpThreshold) {
                chasing = false;

            // attack, if close enough, and if time is OK:
           if (distance < attackThreshold && Time.time-lastFire > attackRepeatTime) {
    var randomness : float = 0.1F;
    var hit : RaycastHit;
    var dist : float = Vector3.Distance (transform.position,target.transform.position);
    var rnd : Vector3 = Random.insideUnitSphere*dist*randomness;

    Debug.DrawLine (transform.position,target.transform.position+rnd,Color.cyan);

    if (Physics.Linecast ( transform.position, target.transform.position+rnd,hit)) {
        if (hit.transform.gameObject == target) {
            //Apply Damage
    if (hit.transform.gameObject != target) {

    //                  target.SendMessage("ApplyDamage",10);
                // Attack! (call whatever attack function you like here)
            if (distance < attackThreshold) {
            //stop when close enough

            //    attackTime = Time.time+ attackRepeatTime;

            // start chasing if target comes close enough


    function OnTriggerEnter (other: Collider) {
    if (other.gameObject.CompareTag("Bullet")){

When using linecasts and raycasts, you can use layermasks to filter what it detects. Read here for an example of how to select and set the layermask.

You will want to put all the child bits into a layer for the purpose of ignoring them.