Donald Glover confirmed in an interview with Fuse that his name came from The Wu-Tang Name Generator, he even spoke to RZA about it. The Wu-Tang Name Generator is a fun computer name app that creates you a rap name based on your real name. Donald entered his name into the generator and thus, Childish Gambino was born.
The Wu-Tang name generator was made in 2002 and if you actually type Donald Glover into the name generator and enter it in, Childish Gambino pops up. http://www.mess.be/inickgenwuname.php
Incase you want to create your own rap name, here is the link.