As a youth baseball coach, you should never teach any advanced pitches like the circle changeup, palmball, forkball, split-finger fastball, or cut fastball. Instead, focus your players on mastering the eventual fastball and natural changeup grips—the two best pitches in baseball. In youth baseball all grips are straight ball grips. I hesitate to say fastball because the pitches don't travel that fast. Let's just say the grips are primarily fastball.
The standard way to grip the ball to throw it with the index and middle fingers on top of the ball with the thumb underneath the ball and the ring and little finger bent and supporting the ball on the side. It is allowable if you have players with small hands to let them throw with the ring finger on top. To pitch you can throw the ball with the seam (2-seam) or against the seam (4-seam).
You can also hold the ball a variety of ways depending on the accuracy and velocity of your pitches. With the 2-seam grip, you can hold the ball in the neck at the seam's narrowest point, at the horseshoe on the top of the 2-seam, or you can place your fingers across both seams that could cause the ball to sink.
With the 4-seam grip, you can hold the ball in the middle of the 4-seams with the horseshoe closest to the middle finger or with the horseshoe closest to the index finger.
The grip that you get the best action on the ball is the grip you should use. You can also put your ring and little fingers on top of the ball for a natural changeup.