Using Sitecore 8.2 and the content tree looks like:

sitecore
|_Content
  |_Home
    |_Products
      |_Product 1
      |_Product 2
    |_Services
      |_Service 1
      |_Service 2

The items Products & Services are of the same template which has a treelist field. The items that can be selected from the treelist should be the children of that Menu item.

For eg: For Products, author should see only Product 1, 2 in its tree list. Similarly, for Services, he should see only Service 1, 2.

How can I write a query for the children path, in the template itself to achieve this. For now I'm doing these for each item at item level.

fast:/sitecore/content/home/products/*  
fast:/sitecore/content/home/services/*