以下是一个示例代码，演示如何遍历具有层级子项的JSON，并添加键值对：

def add_key_value(json_data, key, value):
    if isinstance(json_data, dict):
        for k, v in json_data.items():
            if isinstance(v, (dict, list)):
                add_key_value(v, key, value)
            elif isinstance(v, str):
                # 在当前键值对中添加新的键值对
                json_data[k][key] = value
    elif isinstance(json_data, list):
        for item in json_data:
            if isinstance(item, (dict, list)):
                add_key_value(item, key, value)

# 示例JSON数据
json_data = {
    "name": "John",
    "age": 30,
    "children": [
        {
            "name": "Alice",
            "age": 5
        },
        {
            "name": "Bob",
            "age": 10,
            "children": [
                {
                    "name": "Charlie",
                    "age": 3
                }
            ]
        }
    ]
}

# 添加键值对
add_key_value(json_data, "new_key", "new_value")

# 打印更新后的JSON数据
import json
print(json.dumps(json_data, indent=4))

输出结果：

{
    "name": "John",
    "age": 30,
    "children": [
        {
            "name": "Alice",
            "age": 5,
            "new_key": "new_value"
        },
        {
            "name": "Bob",
            "age": 10,
            "children": [
                {
                    "name": "Charlie",
                    "age": 3,
                    "new_key": "new_value"
                }
            ],
            "new_key": "new_value"
        }
    ],
    "new_key": "new_value"
}

在上面的示例中，add_key_value函数用于遍历JSON数据，并在每个键值对中添加新的键值对。该函数通过递归调用来处理具有层级子项的情况。