Child Natsumi Mizuki

I can already picture how "Loli Natsumi" could have been part of the Getbackers series:
Ginji and Ban are hired by a pharmaceutical company to steal back an experimental drug a rival pharmaceutical company stole from them. After successfully stealing the drug, the two of them head back to the Honky Tonk to wait for their clients to pick up the goods but Natsumi ends up mistaking the drug for a beverage and drinks it. The drug to everyone's surprise causes Natsumi to turn into a kid which results in everyone freaking out in a comical fashion. The clients from the pharmacy arrive to pick up the drug only to be told that their waitress had just consumed it. The clients then pull out guns because as it turns out their company was just as underhanded as their rival company. Shooting breaks out and in the process the clients kidnap our loli Natsumi with the intention to "extract" what's left of the drug from her body.
You guys can imagine how the rest of plot progresses.