How do you say my grandson in Swahili?

There's no single word for "my grandson" in Swahili. You need to use two words:

* Mjukuu wangu

Mjukuu means "grandchild" and wangu means "my".

You can also use mwana to mean "child" and wa to mean "of". So, you could also say:

* Mwana wangu wa mjukuu (literally "my child of grandchild")

This is more literal and emphasizes the generational relationship.