db={"parent":[{"_id":"id_123","name":"name","children":["id_child_1","id_child_2"]}],"child":[{"_id":"id_child_1","name":"child1"},{"_id":"id_child_2","name":"child2"}]}

db.parent.aggregate([{"$lookup":{"from":"child","localField":"children","foreignField":"_id","as":"children"}}])