I Am Using A Hard Plastic Kiddie Pool For A Sandbox And Need To Figure Out How Much Sand To Buy. The Pool Is 4 Foot In Diameter And 12 Inches Deep. How Much Sand Should I Get To Fill It?

Surface Area of pool 3.142X4, equals 6.284, depth of pool is 1 foot therefore cubic capacity of pool is 6.284X1 equals 6.284.
You will need 6.284 cubic feet of sand, a nice round figure would be 6 1/4 cu. Ft.