Firestore data structure

Question

I have a question regarding a database structure. I am using Firestore and plan on building a mobile app using Flutter.

The collection that I want to create would be called something like "items" which would have parent documents as well as children. Children will have a parent value which is the parent's unique id.

This might get very big, children can have their own children, that's why I don't know if having everything in one collection is correct. What is the proper structure for something like this in Firestore?