Following The Legend of Korra, that Toph casually told Lin her father was a nice man called Kanto with whom things didn't work out. Suyin's father, meanwhile, remains unnamed.
Though sequel comics and flashbacks in The Legend of Korra have filled in some of the details of what happened after the events of Avatar: The Last Airbender, much about the later life of Team Avatar remains a mystery. We know that Sokka died at some point before the events of The Legend of Korra, but not how he died. It's never mentioned that he had any children, who would have been cousins to Aang and Katara's three kids, so it's assumed that he didn't. Still, the lack of details has left plenty of room for speculation.
It's easy to understand why some Avatar: The Last Airbender fans think Toph and Sokka would have been a perfect match. Both of Sokka's canon love interests were powerful women: Yue, princess of the Northern Water Tribe, who sacrificed her mortal life in order to protect the city; and Suki, the leader of the Kyoshi Warriors. Toph and Sokka also shared several close bonding moments. For example, Toph had a treasured bracelet made from the same meteorite that Sokka used to forge his sword. The two of them fought side-by-side in the final battle against Fire Lord Ozai, and Sokka shielded Toph's body with his own when it seemed like they were about to die. Then, in the sequel comics, Sokka helped Toph with training her first recruits at the Beifong Metalbending Academy.
Sokka and Toph never actually dated in canon. At the end of Avatar: The Last Airbender, and in the comics that followed, Sokka was dating Suki. We don't know how long that relationship lasted, but we do know that Sokka ended up living in Republic City, where Toph was the chief of police. Avatar fans who ship "Tokka" (Toph and Sokka) have raised the possibility that Sokka could be Suyin's father, pointing to a slight resemblance between Sokka and Suyin's own children. However, based on what we know about Sokka, this seems very unlikely.
Though he was often used for comic relief in Avatar: The Last Airbender, Sokka was also brave, principled, and fiercely loyal. We know that he lived at least into his forties and was living in Republic City at that time, when Lin and Suyin would have been children. For Suyin not to even know who her father was would mean that Sokka chose not to acknowledge and help raise his daughter, which seems very out of character. It could be argued that Toph didn't tell Sokka he was Suyin's father, but even Sokka probably would have put two and two together if Toph had a baby nine months after they dated.
While it's certainly a lot of fun to speculate (and Sokka not being Suyin's father doesn't preclude the possibility of he and Toph having dated at some point), this is one fan theory that would hurt the integrity of Sokka as a character if it were true. Moreover, while Lin Beifong nursed a grudge against Toph for a long time because of never knowing who her father was, Suyin never appeared to be particularly bothered by not knowing her own father - instead focusing on building a family and a legacy of her own. Toph is set to star in her own graphic novel, Toph Beifong's Metalbending Academy, so perhaps that will shed some more light on who Lin and Suyin's fathers really were.
