In soccer, the term "tackling" is used to describe any effort to steal the ball or knock it out. Thus, in soccer, a tackle can be on the ground (a slide tackle). It also can be done upright, either by knocking the ball out as you run along one side of the attacker or done from the front by blocking him from dribbling the ball forward. Unlike American football, a tackle really is not supposed to bring the opponent crashing to the turf. Indeed, in many cases, the tackle will be judged as a foul if you do so. As a result, your first job as a coach will be to explain the terminology to your players and make the point that the player is tackling the BALL ITSELF and not the opponent. Alternatively, you can simply use a term like "ball-stealing", so that they don't have the idea that they will playing American-style tackle football with a pointy pigskin instead of a soccer ball.
The easiest tackle to learn is the knock-out tackle, which is used to hit the ball away when the attacker gets the ball too far in front. Teaching tackling is usually begun right after basic work on First Defender skills and the defensive stance.
Use the same grid and follow the same ideas of patience and delay, but give permission to go in for the ball when the attacker allows it to get out too far in front of him. In this exercise, the defender gets a point for sending the ball out of the grid in any direction while the attacker gets a point by dribbling over the opposing end-line. This tackle is used when you want to buy time for your teammates to get back and is particularly popular with wing defenders.
The next tackle, which is also very easy to learn, is the bump. To do this, quickly move into the attacker from a slight angle, lifting your lead foot over the ball so that your lead foot will end up between his legs. At the same time, turn into him with your shoulder/hips so that you can bump the him off the ball with your backside and take the ball away with the outside foot.
Once you commit, the key is to pounce aggressively and go in hard. The lead foot comes across the ball just when your shoulder/hips are firmly against the attacker. Try to time the move in so that the attacker is momentarily looking down at the ball and is standing on his dribble foot so that he is temporarily frozen. This exercise also can be done in the same grid, using the same procedure as before. One point for stealing the ball and one point if the attacker can get around the defender and dribble across the opposing end-line.
The next tackle is the front, or "block", tackle. The purpose is to block the ball into the attacker's foot, then to drag/lift the ball over the attacker's foot. Body weight must be over the ball to prevent the attacker from getting any leverage.
Key coaching points are: ankle of blocking foot locked, to make foot into an L-shape (hoe-shape) to use as a drag; keep body weight over ball; put support foot well to the outside.
Put the pairs back into their grid, one ball per pair. Have the attacker stop, with the ball just to the inside of his right foot. Now, have the defender step in so that his shoulder presses into the attacker just as his right foot blocks the ball into the attacker's right foot so that the ball cannot go forward. As soon as the block is made, the defender will try to lift the ball to the outside and bring it over the toes of the attacker by getting his locked foot under the ball and lifting/rolling it over. Note that the support leg must be sufficiently wide of the attacker to allow the blocking leg/foot and the ball to come across.
This is a more difficult tackle to learn, so give the players ample time to experiment. Most young players can block the ball easily but have trouble with the technique of dragging it over. Some will try to put the outside leg too far forward, and bump knees and/or give the attacker room to push the ball through their legs. Others will try to put the outside foot too far to the back so that they have no leverage to use against the attacker. It takes some time to figure out the best place to put the plant foot so that the ball is quickly blocked, and then lifted across. Once some success is achieved by both players, start with some very slow dribbling and let the players try to do just a block. When most have made a good block, then let them try to lift the ball across.
Now, simply let the players play 1v1 in their grids, as follows. The players stand at opposite ends of the grid, taking turns on who is the server. After serving the ball to the opponent, they close him down defensively and try to tackle the ball. Play until the defender has made 3 successful tackles, then switch. Do two full rotations.
Now, combine the kids in adjacent grids, and make a hexagon with some cones. Put a player between the cones at every other leg of the hexagon meaning that they will be in basically a triangle. Put the last player in the middle of the triangle, and play keepaway with the outside players restricted to movement only between their cones. This makes their movement more predictable for the defender. Play until the defender kicks the ball out 3-5 times, then switch. Counsel the defender to use patience when he sets an angle to bottle somebody up as his success is guaranteed by the placement of the outside players once he sets his angle properly.