Did Hashirama have a son? In Naruto it us shown that Tsunade is Hashirama's granddaughter. Does that mean she is the daughter of Hashirama's son? So who is Hashirama's son ?
Hashirama Senju and Mito Uzumaki did have an offspring. But it wasn't confirmed officially if they were male or female. 

Also, the series doesn't give importance to Tsunade's parents, as a result of which, they were never revealed. It mainly focused on the relationship Tsunade and Hashirama had between them and her life later on. 

From Hashirama's and Tsunade's Wikia articles respectively:





So yes, Tsunade is Hashirama's granddaughter. But it is not known if it was Tsunade's father or mother who was Hashirama's child. And, nothing is officially declared about them.