[Response to a letter written about little kids who steal food from the pantry and won't admit to the thievery]
Get a hidden camera that comes on when the beam is interrupted. They are used to photograph animals and can be purchased for less than $100.00. Install it secretly and wait for the lying thief to appear.
First, get each one to sign a statement that they will not steal sweets. If one of them steals, wait for a week to see if the others will as well. Them come down on them with a charge of theft and be prepared to show your evidence. Three good spankings will be in order, plus one year with no sweets.