how to explain cisgender and heterosexual to 5 year old?

A good way to start might just be to show the kid that there are all kinds of people and families, and all are normal. :)

You can find some good kids books that include lots of different types of families, including queer parents. You can probably find them at the library, like “THE FAMILY BOOK” or “A Family Is a Family Is a Family” are some good ones. That might be a good starting point.