In the episode Belson's Sleepover, Jake the Dog from Adventure Time (2010) can be seen on Belson's shelf.
Even though much of the supporting child cast are played by actual children, Jeff is the only out of the main kid characters to actually be voiced by an actual kid.
The character Jeff's appearance/design is based of the Minecraft default character, Steve.
Sumo and Belson are voiced by Jason Marsden in the pilot. In the series, they are voiced by Tom Kenny and Roger Craig Smith respectively.