[{"parentName":"Parent555","hash":"Q2Nj1nGtGJCkHT6eygg1L0OrprQkhGd846fdd5","childName":"Child_one"},{"parentName":"Child_one","hash":"2ac616b42d846fdd52002dc9ad7d810f9f8ce9","childName":"Child_two"},{"parentName":"Child_two","hash":"5cd9119ae60a0a58d38999468648c371c50b7a"}]

db.collection.aggregate([{$graphLookup:{from:"collection",startWith:"$childName",connectFromField:"childName",connectToField:"parentName",as:"child_hash"}},{$match:{"parentName":"Parent555"}},{$project:{parentName:1,hash:1,child_hash:{$map:{input:"$child_hash",as:"c",in:"$$c.hash"}}}}])