I love this idea! I was always tempted to get some of those little squeaker shoes for Jack, but I knew they'd only be cute for about 5 min. - and even less than that for those shopping in the grocery store with us. These are a fun alternative, and easy removable and thus, easily silenced. Here's the how-to from Martha Stewart:
Clack out a beat with dancing feet: Bottle caps transform everyday footwear into ear-pleasing tap shoes. Use two same-size metal tops about 1 1/2 inches wide. With an awl, punch two holes through the top of each cap; they should be about 3/4 inch apart. Thread string or twine through the holes so both ends come out on the inside of the cap. Attach string to shoes by weaving through shoelaces and tying securely.