Decimals are a part of the whole for which the whole has been equally shared into a number that is a power of 10 -- 10, 100, 1,000. Start with fractions that already have a denominator compatible with decimals. Show the child the fraction 5/10. Ask them to say the fraction aloud -- five-tenths. Write out a place-value chart -- a columned chart with the value of each column labeled with its value. For instance, the chart would have columns labeled hundreds, tens, ones, tenths and hundredths. Show him how to write the number as a decimal on the place-value chart. Practice with other fractions until he feels comfortable placing the decimal on the place-value chart.
The fraction can be changed to a decimal by using multiplication. If the fraction has a denominator that is not a power of 10, teach the child how to find an equivalent fraction which is a power of 10. She writes the fraction down as the first factor in a multiplication equation followed by the multiplication sign. The next factor is the number 1 written as a big, chunky block letter. Any number can be multiplied by one resulting in the same number. For the product, write 10 as the denominator, the numerator is unknown. Ask her if there is any number that the denominator of the first factor can be multiplied by resulting in the product being 10. If the fraction is 3/4, 10 is not a multiple of four. She keeps searching for a multiple that is a power of 10. Add another zero to 10 making the product's denominator 100. Ask the child if 100 is a multiple of 4. Since 4 x 25 equals 100, it is a multiple of 4. In the chunky number one she drew, write the fraction 25/25 as the second factor. Now, multiply the two fractions -- 3/4 x 25/25 = 75/100. Show her how to write seventy-five hundredths on the place-value chart.
Using a calculator is an easy way to make the conversion. Divide the numerator by the denominator. For the fraction 3/4, divide three by four. The calculator shows the decimal after entering the equal key. Practice with the child by calling out fractions. Provide him with a calculator. Let him enter the division equation into the calculator and read the decimal equivalent to you.
Changing decimals to fractions requires less math skills. Ask the child to say the decimal aloud -- "three-tenths." The denominator is tenths, and the numerator is three -- 3/10. Teach the child the mantra, "Say it, write it, you got it."