main.dart

void main() {
  Map firstMap = { "id": 1, "value": [{"key": "A"}, {"key": "B"}]};

  List valueList = firstMap["value"];

  List<Map> newList = valueList.map((valueItem) => {
    "id": firstMap["id"],
    "value": [valueItem]
  }).toList();
  
  print(newList);
}