Today's episode - Page 2

I don't think Kans knew about Balaram until he was well into his childhood, as Rohini giving birth to him was kept a secret by the Gokulvaasis. Moreover, he was only concerned about the eight children of Devaki as per the akashwaani, not Rohini. When he later found out about Balaram, he may have not considered him a threat as he was only Vasudev's son through Rohini. He did not know about the surrogacy after all, and with Vasudev in jail, he may not have considered Balaram a true threat.