IMNSHO
When you present the revolvers to the recipients, I would round up all of the brats and hold a training session. Tell them that dry firing a revolver just for the heck of it should be avoided whether the nipples are out or in or covered with hose or not.
The nipples should easily come out of a cylinder, so my recommendation is to take them out if you are letting the hammer fall for good reason. But since the revolvers will be presents they should be complete....nipples and all. If you can't keep the brats away from the pistols, there are bigger problems than mushroomed nipples. My Grand-daughter is 3 years of age and she knows the thing in our house she is not allowed to touch.
If you simply cannot be certain that the revolvers will not be dry fired, you may want to file the nipples down low enough so the hammer does not contact them when the pistol is fired. In that way, the nipples don't get mushroomed, but the revolver looks complete.
Then if the recipient decides he wants to fire the pistol, you can get him a set of replacement nipples. You can trade one of the brats for six nipples most days of the week.