I cannot count the number of times I have watched a young player get out on a play when they should have been safe at first, had they simply ran hard. This is a very preventable out, and one that will cost you several runs over the course of the season if you don't nip it in the bud.
Younger players especially tend to see first base as a destination, and they will slow down in order to stop on it, or, even worse, time their run to the bag in order to make a little "hop" and land perfectly on the base. Either of the methods will invariably lead to plays that are far closer than should be, and even to outs.
The concept that you must teach is that first base is simply a checkpoint over which the runner passes. In no circumstance should a batter ever simply stop on first base. They should always be charging at full speed, ready to make the turn to second if the ball is in a gap, or simply run down the line otherwise.
Beware that players who run sraight down the line will often like to turn back to their left on the way back to first base - oftentimes placing them in the field of play, and subject to being tagged out. The dugout drill avoids this.
Have all your players line up behind home plate, and put the first kid into the batter's box, as if he is hitting. On your signal, he runs down the first base line as hard as possible. The first base coach either yells "Go! Go! Go!" indicating the he should advance to second base, or "Dugout! Dugout!" indicating that the player must overrun first base by two or three steps, then tail off to the right as if he is going to the dugout. Make them turn back to their right, then race quickly back to first base, ready to advance.
Repeat this drill periodically throughout the season to make sure that your players do not forget and stop on first base!