So in the original epic, Raja Sudhanva didn't attack Mithila for the heck of it (like it's being shown in the serial). He had originally requested for the hand of Sita (in some versions it says he wanted the Shiv Dhanush), along with many other Kings around the time Sita was of marriageable age. When Janak denied him, and the Kings could not lift the Dhanush, they all got together and attacked Mithila. It is in this battle that Janak kills Sudhanva and gives Sudhanva's kingdom to his brother Kushadwaja. That's how Kushadwaja becomes a King. All of this happened before Ram ever came to Mithila.
Maharaj Janak was not only saintly in character, he was also a great warrior with a mighty army. I hope this show doesn't show Janak defeating the Kings with his "wise lectures", or worse, Sita entering the battlefield in an era where women were not allowed to fight in battles, let alone small children!
I am posting snapshots of the story from two different sources, along with links.
An excerpt from History of Ancient India and its Kings
Source: http://www.amazon.com/History-Ancient-India-New-Version/dp/8126906154
Valmiki Ramayana (Bala Kanda)
Source: http://valmikiramayan.net/utf8/baala/sarga66/bala_66_frame.htm