Start counting with your children when they are small. Count cars on the street, cookies or the stuffed animals in the bed. Introduce the concepts of addition and subtraction using small numbers they can understand. If you count five ducks in the park, then two fly away, ask how many are left. Help them to figure out how many plates to put on the table when Grandma and Grandpa come to dinner, or how many candies they have left after they have already eaten the first three.
Once your children start school they will be able to learn and understand much higher numbers. Continue to use physical objects like coins or matchsticks, so that they can see and understand the quantities involved. Offer incentives: if they correctly figure out how many small candies a sum makes, they get to eat them. Don't forget to keep a paper and pencil on hand and write everything down so that they make the association between physical quantities and written numbers.
Numerous websites offer free addition and subtraction games for children. To help them get the most benefit from them, sit with them and talk about the answers, how they can find them and why they were right or wrong. These games offer incentives in the form of computer time and animated figures or sequences. If you have a child who loves to play on the computer, try turning that to good use.
Most board and card games require some sort of basic number recognition and counting. Incorporate math skills by altering the games a little. For instance, in the card game "war" each person draws one card from his hand, and the highest number wins. Instead, draw two or three cards at a time, with the person who has the highest total winning. In a board game where you would usually roll the dice once and then move, roll the dice twice, adding or subtracting to get the correct move.