Database
[{"_id":"xxx","ParentNumber":"12345","ChildNumber":"A123"},{"_id":"yyy","ParentNumber":"12345","ChildNumber":"B123"},{"_id":"zzz","ParentNumber":"6789","ChildNumber":"C123"}]
Query
db.collection.aggregate([{$group:{_id:"$ParentNumber",Children:{$push:"$$ROOT"},},},{"$project":{_id:0,ParentNumber:"$_id",Children:{"ChildNumber":"$Children.ChildNumber"}}}])