I've taught lessons about tithing in Primary before. The children always enjoy this lesson. We'll talk about all the things people do with money they earn. I'll take a pile of pennies and show the children how to figure out a tithe.
I'll tell the story of the widow's mite. "And Jesus sat over against the treasury, and beheld how the people cast money into the treasury: and many that were rich cast in much. And there came a certain poor widow, and she threw in two mites, which make a farthing. And he called unto him his disciples, and saith unto them, Verily I say unto you, That this poor widow hath cast more in, than all they which have cast into the treasury: For all they did cast in of their abundance; but she of her want did cast in all that she had, even all her living," Mark 12:41-44.
The widow gave only a small amount compared to the amounts that some rich people paid. However, Jesus knew that she gave what she had, because she loved Heavenly Father. We too can show our love for Heavenly Father and Jesus Christ by paying our tithing, even if it is just a small amount.
I'll tell about my son, who has saved his money for the things he wants ever since he was a little guy. They'll like the stories of how he learned to pay his tithing first, then save half for fun and save half for a rainy day.
Tithe means tenth part, given for the service of God. Since everything we have is a blessing from God part, He doesn't ask much in return. One tenth is sufficient to build up His kingdon. Paying a tithe is the duty of every believer and a test of faithfulness to God. These funds are used to build temples, chapels, and other facilities useful for building up His kingdom as well as printing lesson materials.
After this we will draw pictures of ourselves paying our tithing. As they finish, I'll ask each child to hold up their picture and tell us about their interpretation of paying tithing.
It's a simple lesson, but they always get the principle.