To enable a Burndown chart of Epic tickets with child artifacts attached to it, you have to add a planning folder to the epic tickets.
Steps to enable this Chart:
Create some Epic tickets
Add child artifacts to the Epic tickets.
Add a planning folder for all the epic tickets or the tickets that is required for the graph.
Now create a burndown report by selecting the planning folder id of the planning folder selected in the Epic tracker and tracker id of the Epic tracker and save it.
You will be able to view the burndown chart in the project home now.
In the attached screenshots, you will be able to see we have attached a planning folder "Chakchak" and then when creating a burndown report by selecting the Planning folder id of "Chakchak" and selecting the Epic tracker id, the burndown chart is created.