if the family had two or three childrenit was harder to pay for all the child

不是虚拟语气啊，是条件状语从句。had在此是有的意思。

意思是如果一个家庭有两到三个孩子，这将会很难去抚养这么多

主句和从句的时态当然要一致啦，楼下的。主句是过去式，从句肯定也要用过去式

when也可以，但意思会有所不同

虚拟语气
对现在的假设
If the family had 2 or 3 children, it would be harder to pay for all the children.
对过去的假设
If the family had 2 or 3 children, it would have been harder to pay for all the children.
对将来的假设
If the family should have 2 or 3 children,it would be harder to pay for all the children.

句子有错，最后一个词应该是children。
主句和从句一个时态很正常，条件和结果都谈的过去。那时候如果一家有几个小孩（对于孩子不止一个的家庭），养所有的孩子（为所有孩子买....）比样一个更难。
如果是对现在的虚拟，则应该用If the family had two or three children, 现在都是独生子女家庭了，如果有两三个孩子it would (就会)be harder to pay for all the children.

是虚拟语气啊
是对现在的假设，所以用过去时had
如果只是一般的假设，那么就用have ,或者Has了