设置子对象的to_json方法格式

class ChildObject:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def to_json(self):
        return {
            'name': self.name,
            'age': self.age
        }

class ParentObject:
    def __init__(self, child):
        self.child = child

    def to_json(self):
        return {
            'child': self.child.to_json()
        }

# 示例用法
child = ChildObject('Alice', 10)
parent = ParentObject(child)
json_str = parent.to_json()
print(json_str)