Nonna's water pistol appears to resemble a Soviet PSM pistol, based on the shape and the safety selector on the top rear of the slide.

Alice's rocket launcher-looking thing seems to be a jumble random stuff, but the rail and block on the bottom are reminiscent of the rear sight cover and firing pin channel on the back end of an M72 LAW tube.

Chiyo seems to be carrying a water gun approximation of an MP5K "Operational Briefcase" (basically a briefcase with an internal mount to hold a submachine gun and a mechanical linkage connected to a trigger on the briefcase handle).