新增小程序后端代码，包括数据库、路由、服务层等。

小程序前后端打通，包括登录、注册、绑定设备、查询绑定信息，修改小朋友名称等功能。

mini-program/app/service/child.py

@@ -0,0 +1,42 @@
+from collections.abc import Mapping
+from datetime import date
+from typing import Optional
+
+from app.dao.child import ChildDAO
+
+
+class ChildService:
+    def __init__(self, db):
+        self.dao = ChildDAO(db)
+
+    def create(
+        self,
+        user_id: int,
+        child_name: str,
+        child_gender: int = 2,
+        child_birthday: Optional[date] = None,
+    ) -> Mapping:
+        child_id = self.dao.create(user_id, child_name, child_gender, child_birthday)
+        return self.dao.get_by_id(child_id)
+
+    def list_children(self, user_id: int, limit: int = 20, cursor: int = None) -> tuple[list, bool]:
+        rows = self.dao.list_by_parent(user_id, limit, cursor)
+        has_more = len(rows) > limit
+        rows = rows[:limit]
+        return rows, has_more
+
+    def get(self, child_id: int) -> Optional[Mapping]:
+        return self.dao.get_by_id(child_id)
+
+    def update(
+        self,
+        child_id: int,
+        user_id: int,
+        child_name: Optional[str] = None,
+        child_gender: Optional[int] = None,
+        child_birthday: Optional[date] = None,
+    ) -> Mapping:
+        if not self.dao.has_access(child_id, user_id):
+            raise PermissionError("No access to this child")
+        self.dao.update(child_id, child_name, child_gender, child_birthday)
+        return self.dao.get_by_id(child_id)