A fun way to practice plotting points on the number plane is to play Battleship. You will need two pieces of graph paper, two pencils, and two kids. Each kid should draw a number plane on her paper with an x-axis and a y-axis, numbered from -10 to 10. Then draw lines on your paper to represent the battleships. Each kid gets six ships. Two of the ships are little two-space ships. Three of them are three-space ships, and one is a big four-space ship. When you have your ships placed just how you want them, you are ready to start.
Your opponent calls out a coordinate - a y number and an x number, like 3, 4. If that number has a point drawn on it on your sheet, you call out "Hit!" and make an x through that point. If there's no ship there, you call out "Miss!". Either way, it's your turn next to try to hit his ships. You have to hit all of the points on a ship in order to sink it. Whoever loses all of his ships first loses the game.
Don't cheat! Your opponent may be keeping track of all her guesses so she can check them against your ships at the end of the game.