Black powder and BP substitutes still seem available, pick up an 1858 New Army Remington replica and spare cylinders(you can mail order them to boot) and use that for the “fun gun”. Cheap as heck and you can make the balls or bullets (Lee has a conical mold for percussion revolver bullets) from old wheel weights and scrap lead too!
“pick up an 1858 New Army Remington replica and spare cylinders”
BP pistols are nearly impossible to find now and the cost is through the roof.