How can I find _id of 'MMT 13' and push new object to children array?

How can I find _id of 'MMT 13' but don't know parentID of children and push new object to children array ? I use nodejs to do this

This is my code

db.proof_folders.find({"children._id":ObjectId('632e7c26ef844bc6a4b2af5f')})