[{"_id":ObjectId("617d9ed5cfd28c2c5fd8554c"),"title":"Category 1","children":[ObjectId("617d9ed5cfd28c2c5fd8554d"),ObjectId("617d9ed5cfd28c2c5fd8554e")]},{"_id":ObjectId("617d9ed5cfd28c2c5fd8554d"),"title":"Category 2","children":[ObjectId("617d9ed5cfd28c2c5fd8554e")]},{"_id":ObjectId("617d9ed5cfd28c2c5fd8554e"),"title":"Category 3","children":[]},{"_id":ObjectId("617d9ed5cfd28c2c5fd8554f"),"title":"Category 4","children":[ObjectId("617d9ed5cfd28c2c5fd8554c")]}]

db.collection.aggregate([{$lookup:{from:"collection",localField:"_id",foreignField:"children",as:"childOf"}},{$match:{childOf:{$size:0}}},{$project:{_id:1}}])