What sydnie7 said. You need 12 inches of fabric so that you can make a 6-inch deep pleat. Trust me, up there, "deep" is good.

As for kid-size pleats, how about 1 - 1.5 inch? Something like that.

Try this. Measure around Juniors rump. Now add an inch, maybe an inch and a half for diapers and also for "growth room". Divide that number in thirds. One third will be the over-apron and two-thirds will be the pleats. Now, take that two-thirds-rumpus number and divide it by however many pleats you want to go 'round Juniors backside. Seven might be a good number of pleats. What you get is how wide juniors pleats will be.