First chapter of Ramayana, Balakanda mentioned Valmiki narrating the Ramayana to his disciples Lava and Kusha. But their birth and childhood story is mentioned in last chapterUttara Kanda which is not believe to be original work of Valmiki. According to the legend, a pregnant queen Sita leaves the kingdom of Ayodhya when she comes to know that Ram was anxious when he heard a washerman of Ayodhya questioning Sita's purity. She then took refuge in the ashram of the sage Valmiki located on the banks of the Tamsa River. Sita gave birth to twin sons, Lava and Kusha, at the ashram and were educated and trained in military skills under the tutelage of Sage Valmiki. They also learned the story of Rama.
Ashwamedha Yagya
During an Ashvamedha Yagya held by Rama, Sage Valmiki along with Lava and Kusha, attended with Sita in disguise. Lava and Kusha chanted the Ramayana in the presence of Rama and a vast audience. When Lava and Kusha recited about Sita's exile, Rama became grief-stricken and Valmiki produced Sita. Sita, struck with embarrassment and grief, called upon the earth, her mother, to receive her and as the ground opened, she vanished into it. Rama then learnt that Lava and Kusha were his children. In some versions, Lava and Kusha capture the horse of the sacrifice and defeat Rama's brothers and their army and when Rama came to fight with them, Maharishi Valmiki intervened and then Sita discloses to their sons that there father is Lord Rama and Valmiki told the children that through song they have to recite the story of holy Ramayana to the people of Ayodhya that how Sita sacrifices even Gold Lanka to wash away the spoilt image of Sita from Ayodhya people.
Later history
Lava and Kusha became rulers after their father Rama founded the cities of Lavapuri and Kasur respectively. Cantos sixteen to nineteen of the Ānanda Rāmāyaṇa describes the exploits of Rama's progeny. The manifestation of a Goddess appears before Kusha, declaring to be the Tutelary deity of the ancient capital of Ayodhya. She described the condition of the deserted city, which had been abandoned and ruined since the departure of King Ram. Kusha sets forth with his whole army to restore the city to its former splendor. Cantos sixteen describes Kushas marriage with the Naga Queen Kumudvatī. While residing in Ayodhya in the summer, Kusha goes to the Sarayu to bathe with the ladies of the court. While sporting in the river, he loses a great gem bestowed to him by his father. In anger Kusha threatens to shoot an arrow into the river, whereupon the river parted revealing Kumudvatī. Cantos seventeen describes the final years of Kusha. Kusha and Kumudvati have a son named Atithi, who becomes heir to the kingdom. Kusha engages in a battle with a demon and gives his life in the process of slaying his adversary. Cantos eighteen and nineteen describe the 21 Kings that succeeded Atithi. Kushavati was a city in Kosala Kingdom as related in the epic Ramayana. The king ofKosala, Rama, installed his son Lava at Sravasti and Kusha at Kushavati.