The basic concept behind decimals relates to our system of counting by tens. Before your child can begin to understand decimals he must be able to grasp the differences and relationships between ones, tens, hundreds and thousands, and also between tenths, hundredths and thousandths. Emphasize that a decimal point always separates the ones from the tenths. One visual way to do this is with blocks or squares to represent relative values. For instance, a 10 block would be ten times as long as a 1 block.
Once your children have grasped the concepts of how numbers are multiplied or divided by tens, teach them that each digit, or decimal place, has a proscribed value. In other words, there's a place to put your hundreds, a place to put your tens, a place to put your ones, your tenths, your hundredths, et cetera. Commas and decimal points help keep track of the different place values.
One good way of illustrating the principle of decimals is to use money. It is a familiar, meaningful medium always written as decimals. Furthermore, it gives you something tangible to work with. Help your children practice counting various amounts of bills and coins and then figure out how to write the total down properly. Explain how messing up the location of the decimal point results in losing or gaining a lot of money. For instance, if you have $23.60, but write it as $2.36 you end up losing $21.24. Or if it becomes $236.00 you have now "stolen" $212.40.
For visual children it may help to imagine the decimal point as a center mark. On the left side of the center mark, the numbers get larger and larger the further away they get. On the right side of the mark, the numbers get smaller and smaller. 200.0 is bigger than 50.0 because it's further away from the point. On the other hand, 0.009 is smaller than 0.01 because it's further away on the other side.