I knew that's what you meant when first asking about lighter springs and right you are, the short hammer lacks the leverage, especially for kids. You can cut off a few coils or buy aftermarket lighter hammer and trigger springs like Wolff from brownells, Midway, etc. Not expensive.
Now that I know the guns are new and unmolested, I suspect the hammer/transfer bar function is the cause of the misfires.
TO TEST: Cock and dry fire the gun but do not release the trigger. While still holding the trigger back, observe how far the firing pin is sticking out thru the recoil plate in the breechface. Now cock the hammer and using something with a flat tip like a punch, push the rear end of the firing pin in flush with the frame in the hammer slot. While holding it there, observe the firing pin again. Does it stick out slightly further? If so, the front top pad on the hammer is too long (thick) which doesn't allow the transfer bar to push the firing pin flush with the frame. File the pad down until you get full firing pin protrusion thru the recoil plate while holding the trigger back. This is not a common problem but shows up once in awhile. Particularly when hammers are swapped.
To make sure the hammer is still safe, when you go to the range, test it like this: load the cylinder as usual with the hammer on halfcock. Now with gun pointed safely downrange, cock the hammer all the way back. While holding hammer, pull the trigger and ease the hammer down just past the 1/2 cock position. Take finger off the trigger and let the hammer fall. The gun should not fire. And there should be no evidence of a light firing pin strike on the case rim. The hammer safety design is functioning properly.
See more here: http://www.cylindersmith.com/Transferbar.html
For smoothing the action, the key things to check are, with hammer spring removed, the hammer revolving freely on the hammer screw; also moves back and forth freely in the frame with screw tight.
There's more you can do but just go as far as you feel competent:
A fine Arkansas sharpening stone is the professional way. But 1500 followed by 2000 grit paper on a hard (steel or glass) perfectly flat surface will do. Stay away from Dremel and other power tools. Retain all original angles when polishing critical surfaces like the trigger sear and hammer notches. Magnifying glasses and excellent light are a must to see your progress! If you don't have gunsmith glasses, you can get the 5 power specs at the drugstore for $2.
Look at all contact surfaces for evidence of the factory machine marks and focus on polishing those out. Working on a gun with some use on it is an advantage because you can see the spots where the parts rub together. Don't take off any more metal than absolutely needed to get the surfaces smooth. Also the two pins that the hammer, trigger and cylinder bolt revolve on.
