Why would we need to drop another bomb to show Russia and China what we could do? That doesn't even make sense accidentally.
Russia, not China. Mao hadn't destroyed the Nationalists yet and no one really yet knew that the Red Army had basically sat out much of the big fighting with imperial japan.
and again, no one knew what Hiroshima was. early japanese reports indicate that high command just thought that someone had bombed the communication link with the city.