[{_id:1,property:"a",children:[{_id:2,property:"b"},{_id:3,property:"c"}]}]

db.collection.aggregate([{"$project":{"_id":0,"children":{"$concatArrays":["$children",[{"_id":"$_id","property":"$property"}]]}}},{"$unwind":"$children"},{"$replaceRoot":{"newRoot":"$children"}}])