[{id:"ID1",name:"ABC",child:[{"key":"key1","val":"val1"},{"key":"key2","val":"val2"}]},{id:"ID2",name:"XYZ",child:[{"key":"key1","val":"val1"},{"key":"key3","val":"val3"},{"key":"key4","val":"val4"}]},{id:"ID3",name:"PQR",child:[{"key":"key1","val":"val1"},{"key":"key2","val":"val2"},{"key":"key4","val":"val4"}]},{id:"ID4",name:"PQR",child:[{"key":["key1","key2"]},{"key":"key2"}]}]

db.collection.find({child:{$elemMatch:{key:{$all:["key1","key2"]}}}})