Use money as an entry point. Third-graders have learned about money and know how to add coins to total a dollar. Using a dime to represent tenths of a dollar can illustrate that there are numbers between whole numbers. Stick to using dimes; introducing other coins and other place values may confuse many in your class.
Build a pretend store with toy groceries or real canned goods. Price each item with a sticker, again keeping prices in the tenths place. Give the children a total price and have them gather products that add up to that price, adding the correct amount of tenths. Have them use play money to pay for their purchases.
Use worksheets to have kids add decimals and whole numbers to help them understand that decimals can add up to whole numbers. For instance, have them add 2.3 and .9 and explain how it adds up to 3.2. Have the students fill out the worksheets until they have the concept firmly in their grasp.
Teach kids to convert decimals in the tenth place into fractions to give them a clearer idea of partial numbers. Use worksheets and have them work the problems, such as converting .7 to 7/10. These steps will prepare kids to move on to the hundredth and thousandth places.