First consider the basic principles of a rocket liftoff.

Any rocket moves as a direct result of Newton's Third Law of Motion (equal and opposite reactions). The rocket "throws" gases toward the ground (either hot exhaust gases or, in this case, pressurized air), and the rocket experiences a force in the exact opposite direction. Because the force on the rocket from the gases is directed opposite the direction in which the gases are thrown, then ideally the gases are being thrown in the direction exactly opposite where the rocket should go. For example, for a vertical liftoff the gases should be directed straight downward, thereby pushing the rocket straight up into the sky.

Now imagine that the rocket is moving. The air around the outside of the rocket is also exerting forces on the rocket. (Really this could be occurring before launch as well - wind still blows even when the rocket is still on the launch pad. The rocket is typically stabilized at this point though, and the fins are not important.) These forces alter the direction of the rocket and cause it to rotate and point somewhere other than intended. The purpose of the fins is to produce additional forces that corrects for these perturbing effects and thereby keep the rocket on a straight path. When the rocket is rotated away from the intended direction, the fins are placed into the passing stream of air. This passing air exerts a force on the fins in the opposite direction, causing rotation toward where the rocket was originally aimed (for this reason the force from this air is called a restoring force). The amount of force exerted on the fins is related to the area available for the passing air to push against: more fin area means more for the air to push against and a larger restoring force.

This is not the only consideration though. Realize that the fins must be made of some kind of matter, meaning they have mass. Like all mass, this experiences a force toward the earth due to gravitational attraction. Since the rocket is probably being launched away from the surface of Earth, this force opposes the force from the propulsion gases. Fins also cause aerodynamic drag.

Aerodynamic drag is sort of like friction from air. It acts opposite the direction of motion of the object, so a rocket moving away from Earth's surface will experience a drag force acting toward the ground. There are several sources of drag, with the one relevant to this question being the geometry of the object causing the drag (here, the fins).

These are the primary aspects when increasing the number of fins on a rocket. Use this information to try to decide the following:
What will more fins do to the ability of the rocket to maintain a steady orientation? Given a certain amount of thrust from the launching gas, what will be the effect of the extra weight from more fins on how the speed to which the rocket can be launched? If adding more fins increases the backward-acting drag force, how will the maximum possible distance be affected?

[Information generally taken from here and here, but neither is really at a 5th grade level. This is another interesting page, but also more advanced. If possible, read it with your teacher].