Sub products

I am looking for a way to create sub products that will show as a dropdown list after a product parent has been added to the cart.

So there will be parent product and child products. Child products only show in the cart after a parent product has been added and you can add a child product directly on the cart page.

I will like to know what is the best way to implement a parent child system and relationship and have the ability to add child product directly to the cart on the cart page

e.g.

-------------------------------------------------------PRODUCT-----------PRICE---
---Parent-Prodruct=>-HP-Computer-System-Unit---|-----------------||-----$250----
related-Child-product-1=>-HP-mouse---------- ----|-ADD-TO-CART--||-----$5-------
related-Child-product-2=>-HP-keyboard------------|-ADD-TO-CART--||-----$25------
related-Child-product-3=>-HP-mouse-pad----------|-ADD-TO-CART--||-----$2-------

The child product will be a drop down and will show directly under the parent product in the cart display
How can this be achieved in ubercart