This is my mum and my dad .对吗？

不对，应该是：
These are my mum and my dad.
或者：
They are my mum and my dad.

This is ……是介绍人物常用语，但只适用于单数，如果要介绍两个人，则要变为复数These are ……
所以上面的说法不对，应改为：These are my mum and my dad

如果前面要用this is 因改写成this is my mum and that is my dad或者说成these are my parents