It's actually from Gravity Falls, not Regular Show, and it's an adorable show whose fans have apparently not yet taken to calling themselves Brovities or something equally retarded. You can get the first episode for free on itunes, and of course there are rips available on Youtube.
That particular scene comes from the fifth episode, "The Inconveniencing".
A post by Jimmy, way back in April wrote:The song is used in MAP04 of Tei Tenga and has the lumpname "LULLABY". I'm fairly certain it's a MIDI of a real song (since most of Tei Tenga's soundtrack is) but I don't know which one.