[{"children":[{name:"Zak"},{name:"Bob"},{name:"Sharon"},{name:"Zak"},{name:"Cindy"},{name:"Bob"},{name:"Peter"}]}]

db.collection.aggregate([{$project:{uniqueCsv:{$reduce:{input:{$setUnion:"$children.name"},initialValue:"",in:{$concat:["$$value",{"$cond":[{"$eq":["$$value",""]},"",", "]},"$$this"]}}}}}])