Magento 2 how to add custom Methods set and get for the **product.php** from our custom module ?

**For example:-** 

    public function getChildernData();
    public function setChildernData();
              

since i have tried in magento 2.2.1 version in the vendor files as mentioned below.

    /vendor/magento/module-catalog/Api/Data/ProductInterface.php
    
    /vendor/magento/module-catalog/Model/Product.php