An absolute rule for a bobbled ground ball is to use the throwing hand to pick up the baseball.
Once a ball is bobbled, athletes will have a tendency to use their glove to try and pick it up.
The chances of dropping the baseball again are much higher when using the glove.
Instruct a "boot and barehand" philosophy!
Take time during fielding instruction to practice bobbling a ground ball and using the throwing hand to pick up the baseball and execute a throw.
For more information, check out my book Coaching the Beginning Pitcher.