[34] => Array
        (
            [sum] => 13
            [name] => 油区开发部
            [id] => 27
            [parentid] => 1
            [children] => Array
                (
                )

        )

    [35] => Array
        (
            [sum] => 2
            [name] => 三分公司
            [id] => 31
            [parentid] => 1
            [children] => Array
                (
                    [0] => Array
                        (
                            [sum] => 156
                            [name] => 项目管理分中心
                            [id] => 52
                            [parentid] => 31
                            [children] => Array
                                (
                                )

                        )

                    [1] => Array
                        (
                            [sum] => 12
                            [name] => 弯管机组
                            [id] => 54
                            [parentid] => 31
                            [children] => Array
                                (
                                )

                        )

                    [2] => Array
                        (
                            [sum] => 20
                            [name] => 车队
                            [id] => 53
                            [parentid] => 31
                            [children] => Array
                                (
                                )

                        )

                    [3] => Array
                        (
                            [sum] => 10
                            [name] => 维修机组
                            [id] => 55
                            [parentid] => 31
                            [children] => Array
                                (
                                )

                        )

                    [4] => Array
                        (
                            [sum] => 55
                            [name] => 机关
                            [id] => 51
                            [parentid] => 31
                            [children] => Array
                                (
                                )

                        )

                )

        )

    [36] => Array
        (
            [sum] => 56
            [name] => 五分公司
            [id] => 32
            [parentid] => 1
            [children] => Array
                (
                )

        )

如何计算三分公司的sum值，是三分公司 children下的 所有 sum的和