I would strongly agree with those suggesting a revolver of some kind, either single or double action. Different kids have different levels of maturity and attention spans but a handgun that automatically readies itself for the next shot, touched off by a light pull on the trigger, might be a bit too advanced for a 6 year old, even under close supervision. You know your kids better than anyone else here so you’d have to be the judge of that. But the idea of learning to fire one deliberate shot can never be instilled too deeply at any age.
Plus you will want to keep the weight down as much as possible. Experience teaching grown women to shoot handguns showed me they don’t like heavy guns – Smith K and J frames were what the grownups liked to shoot so a child’s smaller hands and even lesser strength should be considered. The S&W kit gun is a good choice but pricey these days. The Bearcat with adjustable sights is another great choice and sized right for small hands, blued ones are running from about $450 up on gunbroker.
I'm guessing at those ages the girls haven't become handgun snobs yet (but who knows?
) so a decent used H&R or a Taurus might be a good choice to keep prices down.
