During the half of the end credits, the Banana Splits sing their theme song. At the very end, someone (Patrick Stump pretending to be one of the splits) quietly says "We killed so many people." and we go to a scene with two fingers and a rat.
During the first half of the end credits, the Banana Splits "perform" their theme song. (Though it's a cover by Fallout Boy's Patrick Stump who also composed the instrumentals throughout the movie)
There's no actual opening credits, instead it's an intro of how the original show would've open up as.