I think That Mako and Bolin are Zuko's grandchildren. I think this because, if you look at the picture that Mako and Bolin's dad had sent to thier grandmother, you can see that they are wearing fancy clothes. Yes, they could only have that one set of fancy clothes for a special occasion, but for the sake of this argument, let's assume that they wear these clothes more often. Even Mako and Bolin's cousin thought that they lived in a "huge mansion".
In Cannon, it is stated that Zuko has a daughter, who is the current Fire Lord. I believe that Zuko and Mai got married nd started a family. As we have seen with Aang and Katara's son, Bumi, It is possible for two benders to have a non-bending child. This means that Zuko and Mai's child could have been a non-bender, as Mai has not been shown to be able to bend. I believe that the fire lord can only be a fire bender. Zuko and Mai obviously had one daughter, the Fire Lord after Zuko. But, it is very possible that they had a second daughter, who was a non-bender. This daughter probably met Mako and Bolin's father and the two decided to marry.
Refering back to the package that Mako and Bolin's grandmother got, in the letter, it said that he met a "nice fire nation girl". There was no elaboration.
In the comics, there is a thing called the "New Ozai Society". Mako and Bolin's dad probably didn't add any elaboration on his wife because the society could intercept the message and use Zuko's daughter or grandchildren against him.
Mako says that a firebender killed his parents when speaking with Korra. The NOS (New Ozai Society) would probably have multiple fire benders. These fire benders decided to kill Mako and Bolin's parents and loot their house. Zuko and His other daughter, along with the young Iroh 2, would probably think the boys dead.