Get All simple product from a Configurable Product in Magento Product View

use the following script in

app/design/frontend/default/[your theme]/template/catalog/product/view/type/options/configurable.phtml

Inside the script:

spConfig.getIdOfSelectedProduct = function () {
    var existingProducts = new Object();
    for (var i = this.settings.length - 1; i >= 0; i--) {
        var selected = this.settings[i].options[this.settings[i].selectedIndex];
        if (selected.config) {
            for (var iproducts = 0; iproducts < selected.config.products.length; iproducts++) {
                var usedAsKey = selected.config.products[iproducts] + "";
                if (existingProducts[usedAsKey] == undefined) {
                    existingProducts[usedAsKey] = 1;
                } else {
                    existingProducts[usedAsKey] = existingProducts[usedAsKey] + 1;
                }
            }
        }
    }
    for (var keyValue in existingProducts) {
        for (var keyValueInner in existingProducts) {
            if (Number(existingProducts[keyValueInner]) < Number(existingProducts[keyValue])) {
                delete existingProducts[keyValueInner];
            }
        }
    }
    var sizeOfExistingProducts = 0;
    var currentSimpleProductId = "";
    for (var keyValue in existingProducts) {
        currentSimpleProductId = keyValue;
        sizeOfExistingProducts = sizeOfExistingProducts + 1
    }
    if (sizeOfExistingProducts == 1) {
        alert("Selected product is: " + currentSimpleProductId)
    }
}

Now add onchange event to your dropdown in same page :

onchange = "spConfig.getIdOfSelectedProduct()"

Full description

Use this below code

Code to get the the full product information (where 3 is the configurable product Id)

$product = Mage::getModel('catalog/product')->load(3); 
$childProducts = Mage::getModel('catalog/product_type_configurable')
                    ->getUsedProducts(null,$product);

foreach($childProducts as $child) {
    print_r($child->getName());  // You can use any of the magic get functions on this object to get the value
}

Another code to get the Children Product Ids

$childProducts = Mage::getModel('catalog/product_type_configurable')
                    ->getChildrenIds(3);

Hope this helps!!

A configurable product can have multiple other products associated to it.

Here is the code to fetch all the children products that are associated with a configurable product.

Here goes the code :)

/**
 * Load product by product id
 */
$product = Mage::getModel('catalog/product')->load(YOUR_PRODUCT_ID);

/**
 * Get child products id and such (only ids)
 */
$childIds = Mage::getModel('catalog/product_type_configurable')->getChildrenIds($product->getId());

/**
 * Get children products (all associated children products data)
 */
$childProducts = Mage::getModel('catalog/product_type_configurable')->getUsedProducts(null,$product);

Source: http://blog.chapagain.com.np/magento-how-to-get-all-associated-children-product-of-a-configurable-product/