Der Kinderlumper is a mythical monster from Drusselstein's folklore. The story of the Kinderlumper seems to have two versions. In one version, the Kinderlumper is a kind, loving, creature who brings presents in the night, similar to Santa. The other version depicts him as a ghoul that kidnaps children and puts them in his "gundersack" if they're naughty, or really for any reason, such as blinking, breathing, needing to use the bathroom, having saliva in your mouth, or drinking a glass of water. His powers seem to be coming out of toilets, snakelike flexibility, and shooting bats from his cane.
Heinz Doofenshmirtz was told the negative version as a child while his brother Roger heard the positive one. This is meant as another dig at how much more loved Roger was by their mother than Heinz.
Der Kinderlumper is a huge, troll-like creature with a nose like a broom handle and a huge sack over his shoulder. He looks vaguely like Heinz with a long beard.
"Kinderlumper" seems to be the match-up of the German word "kinder", meaning "children", and "lumper", which is someone who unloads cargo.
He could possibly be based off of Krampus, a creature in German folklore which would torment naughty children during the Yule season.
Kinderlumper may be a play on the word "Kinderräuber", which means "kidnapper" in German. This would match Heinz's story.