They work, but.......
Childrens roll caps, if you can still find them, are no where near as hot as a normal number 11 cap. They produce lots of paper debris that clogs up nipples making using a pick on every shot a need. If you hunt around you can find plastic caps made for toy guns that will fit a number 11 nipple already that are basically the same thing you will be making. They used to come 144 on a card for about 79 cents. We used to use them for target use with the nipples bored out a little. The one real cure for the price of caps is a flintlock, and even flints are not cheap!