The booty is unfortunately planted in Ohio. *sigh* *boo-hoo*
Oh, where was I????
hmmmmm.....kindergarten.....hmmmmm..... Take the pretty blue paint and smear it on the wall in a wavy fashion on the bottom edge, like it's a wave??? it'll work best if there is an extremely faint pencil line to use as the median mark on your wave.
Or the arches? If you want your border to be approx 2 feet high, measure down from ceiling 2' in center of wall, and measure down 2.5 feet at corners. Then a nice smooth arch connecting the 2.5 marks with the 2' mark, and you have a fake domed ceiling look.
Make one of your kids do the measuring and the drawing. 'tis good for the math skillz!