The purpose of Parent & Child OKRs is to link Objectives to each other, based on scope of the Objective.

Suppose that "Improve Application Performance" is a corporate OKR (parent), then "Minimize Bugs" is a team OKR (child). 

That is, a certain chain is built between Two objectives - the larger objective is the parent OKR, and the smaller objective is the child. 

To link objectives, go to "Goals and OKRs" section.

Choose the OKR you need.

Go to the 'Info' section.  

You can find the Parent and Child tab. To link the objective, click the plus sign. 

You need to simply choose any OKR from the list. Or create "New template".

It will appear here.