Children above 12 are able to take part in skiing or other activities_designed_for them.

这里activities designed是直接可以表被动，还是activities which were designed for them省略which were

activities which were designed for them可省略which were，用过去分词作定语表被动

是省略了后者的

后者是正确的。你说的是which引导的定语从句。其实是过去分词作定语。

activities designed是直接表被动的，是过去分词做后置定语；当然，用which were designed for them也是对的，它是定语从句。两者具有相同的语法功能，都是做定语。 【俊狼猎英】团队为您解答。