A Tiny or smaller creature that enter's an opponent's space, be it for attacking or anything else, usually triggers an attack of opportunity. But I don't think charges usually trigger an attack of opportunity from the defender. What about a charge from a Tiny or smaller creature, which needs to enter the opponent's square to complete its charge?
Yes, they would provoke an attack of opportunity for a charge attack against a creature with Reach: 5 feet.
The tiny, diminutive or fine creature makes its charge (against a small or larger creature), but then has to enter the opponents square to attack1, provokes an attack of opportunity, and is then allowed to make it's attack2.
Creatures that take up less than 1 square of space typically have a natural reach of 0 feet, meaning they can’t reach into adjacent squares. They must enter an opponent’s square to attack in melee. This provokes an attack of opportunity from the opponent.
After moving, you may make a single melee attack.