db={accounts:[{account_id:1,name:"abc"},{account_id:2,name:"test"}],parent_child:[{CHILD_ID:23,NAME:"test child"},{CHILD_ID:34,NAME:"test1"}]}

db.accounts.aggregate([{$group:{_id:0,accounts:{$push:{account_id:"$account_id",name:"$name"}}}},{$lookup:{from:"parent_child",let:{},pipeline:[{$project:{account_id:"$CHILD_ID",name:"$NAME",_id:0}}],as:"parent_child"}},{$project:{res:{$concatArrays:["$accounts","$parent_child"]},_id:0}}])