Creating Products as child objects in custom object

Hi,

We have a requirement to create a custom object.  We will be using the custom object to populate the inventory of an account that we capture in a mobile app and send to CRM thru web services. 

I need to do the following:

1. Create the custom object

2. Associate the custom object with the account

3. Add products to the custom object

4. Allow for the creation of multiple custom object inventory records per account

I have created the custom object and can see products as a child of custom object 7 and 9.  What I can't seem to do is add product as a child record of the custom object.  Any advice would be appreciated.