One of the simplest ways to teach children the concept of decimals is to work with money, which is based on the decimal system. The dollar should represent the whole number, whereas different coins represent parts of the whole: a quarter is 0.25, a dime is 0.1, a nickel is 0.05, and a penny is 0.01. This illustrates the basic idea of decimal numbers as parts of a whole. It's also important to teach children how to work with decimals.
First, explain to the student that decimal place values progress opposite to normal place values: each number is successively a lesser value; the first number after the point is tenths, the second is hundredths, and so on. Also show that when adding or subtracting decimal numbers, the decimal point needs to be lined up, and add zeroes to the end of the decimal numbers if necessary. Also, emphasize that a zero after the end of a decimal number does not change its value.
A more visual method to illustrate the concept of decimals is to take a square grid divided into a hundred smaller squares, and explain that the whole square represents the idea of a whole. Each smaller square illustrates a hundredth part of the whole.