学会正确使用 JOIN 语法！
从查询开始，以获取具有最多子级的父级：

select top (1) with ties parent, count(*) as cnt
from family f
group by parent
order by cnt desc

然后加入到 family 获取更多信息：

select f.*
from family f join
     (select top (1) with ties parent, count(*) as cnt
      from family f
      group by parent
      order by cnt desc
     ) pc
     on pc.parent = f.pid

显示 first_name 或者 last_name 对于父级，可以执行以下操作：

SELECT w.Parent, MIN(w.first_name) AS first_name, MIN(w.last_name) AS last_name, count(*) AS 'Number of children' 
FROM family w, family m 
WHERE w.Parent = m.pid 
GROUP BY w.Parent 
HAVING count() = (
    SELECT MAX (mycount) 
    FROM (
        SELECT count(*) mycount FROM family GROUP BY Parent
    ) 
a);

只需加入（正确）和分组：

select top (1) P.First_Name, P.Last_Name
    , count(*) [Number of children]
from dbo.Family P
inner join dbo.Family C on C.Parent = P.id
group by P.First_Name, P.Last_Name
order by [Number of children] desc;

然后对于从中显示的每个附加列 P ，按同一列分组。