Ruger makes a SP101 in .22 with a 4" barrel and a Hi-Viz front sight and an adjustable rear sight. It will cost about $230.00. The manual of arms is very simple; the sights are good and the sight radius is long enough for precise shooting. I have always felt that a revolver, with the simplified manual of arms, is the best bet for beginners (and old timers like me). They almost never have any feed or extraction issues like an automatic might. An identical version of this Ruger is available in .357/.38 at the same price.
The .22s make the most sense to me for the two kids. If each shot 50 rounds per session (100 total rounds) you would be talking about $2.00 to $4.00 in ammo. If you did the same with a low cost new-manufacture .38 it would cost about $27.00. In 9mm about $18.00.
And kids like to burn through a lot of ammo, so a brick of 500 rounds is about $30.00. So the .22 is not likely to break the bank. Light recoil too.