After Tang San defeated Qian Renxue in the last part of the Douluo Continent, he didn't send Dai Mubai to the front line. On the contrary, he returned to the Huo family after this victory, preparing to leave the hobbit world with Xiao Wu. Dai Mubai wasn't sent to the front line in the novel because he wasn't Tang San's biological son. Dai Mubai's identity was the Hobbit world's Bitman. His existence was to pass information between the Hobbit world and the human world to assist Tang San in completing his mission. So Tang San didn't send Dai Mubai to the front line, but rather let him stay in the hobbit world to continue serving the mission.