你可以使用Python中的列表推导来过滤子对象的长度。以下是一个示例代码：

data = [
    {"name": "Alice", "age": 25, "hobbies": ["reading", "running", "swimming"]},
    {"name": "Bob", "age": 30, "hobbies": ["playing guitar", "painting"]},
    {"name": "Charlie", "age": 35, "hobbies": ["cooking"]},
    {"name": "Dave", "age": 40, "hobbies": []}
]

# 过滤出hobbies列表长度大于等于2的子对象
filtered_data = [obj for obj in data if len(obj["hobbies"]) >= 2]

print(filtered_data)

以上代码将输出：

[
    {"name": "Alice", "age": 25, "hobbies": ["reading", "running", "swimming"]},
    {"name": "Bob", "age": 30, "hobbies": ["playing guitar", "painting"]}
]

在代码中，我们使用列表推导来遍历列表中的每个子对象。然后，我们使用len函数来获取每个子对象的"hobbies"列表的长度，并使用条件if len(obj["hobbies"]) >= 2来过滤出长度大于等于2的子对象。