原句有误，似改为：

The children are to do their homework the moment they _______ at school.

名词词组 the moment 可以用作连词，引导时间从句，意思跟as soon as 同。

主句是 be to do 将来时，时间从句用一般现在时。若没有，则可以考虑使用现在完成时，代替将来完成时，表示将来那个时间业已完成的动作。

故本题选择 A。

语法知识：

现在完成时用在“时间状语从句”或者“条件状语从句”中，表示将来业已完成的动作或状态，代替将来完成时。例如：

1. 时间状语从句：until, as soon as, when

We can't move in until the electricity has been laid on.  我们得等电源接通后才能搬进去。

I'll go to my granny's as soon as I have finished my homework. 我一完成作业就去奶奶家。

Don't forget to switch off when you have finished.  做完之后别忘了关掉。

2. 条件状语从句：if, once, as long as

If you have finished reading the book before I leave, please lend it to me. 如果你在我离开之前就读完了这本书，请借给我。

Once we have promised, we should keep it. 一旦许下诺言，我们务必遵守。

You can watch TV as long as you have finished your homework. 只要完成了作业，你就可以看电视。