Is there a reason why a 2 year old child would have an anal fissure?

Most common. Is constipation. As the large hard stools pass through the anal opening it may cause a tear, that is the fissure.
Big/bulky/fast. The anus is like a purse string. The tissues stretch out to allow stool to pass. If the stool diameter is too large , has big lumps or pushes out too fast, the event will tear the tissue & create a fissure.