@Anna Koshelkova - As you have found out for yourself, the hierarchy is lost on export. There is a Wish List Idea to capture the hierarchy on export, so I suggest that you add your comments/vote to:

https://community.miro.com/community-stories-60/cardsy-create-cards-in-bulk-from-selected-widgets-on-the-board-679

Workarounds?

• Tags? Nope. I was originally going to suggest using tags, but the mind map nodes don’t accept tags (and you’d be creating lots of tags).
• Comments? Nope. While they attach to the nodes, they don’t appear in the CVS export.
• Convert nodes to Cards? Yes and no. I tried using @Yash‘s Cardsy web-plugin and while the nodes convert to cards, the links are lost. However, this is still not a terrible option. However, you’ll have to tag all of the cards with their corresponding mind map hierarchy.
• Miro Developer Platform - Maybe. You could investigate what information/meta data is available for the mind map nodes using the Miro SDK. However, looking back at this post from 7 months ago, it was not possible at that time - perhaps something has changed. 
• Adding your own meta data to node names - Not elegant, but it would work. In this example I prefixed all nodes with “R” for root and then “P#" for each parent and “P#C#” for each child:

And then used a combination of the LEFT and FIND Excel formulas to extract the node’s location:

=LEFT(A2,FIND("_",A2,1)-1)