According to the IAF we don't know. What we do know is that Kfar Kana was used to shoot rockets at Israel. As Warner Wolf used to say "let's go to the videotape". The IAF played a video which showed rockets being launched from that building in Kfar Kana.
The IAF said as follows. The building in question was attacked between 12:00 and 1:00AM, it did not collapse until around 8:30AM. A further attack around Kfar Kana took place at 7:30AM but it hit 3 different buildings and based on the camera evidence all the bombs hit their targets and the building in question was not hit.
Therefore, the IAF at this point is not taking responsibility for what happened. It is very possible that sometime in the morning, Hezbollah did something that caused the building to explode.
Here you can see a video taken from the air of Hezbollah shooting rockets from right next to Kfar Kana and then driving right into the village.
In addition we need to ask the following very simple question, why were so many children in a war zone, even though the population had been warned by the IDF to leave several days earlier?
Leaflets distributed by the IDF, July 25, 2006:
To all citizens south of the Litani River
Due to the terror activities being carried out against the State of Israel from within your villages and homes, the IDF is forced to respond immediately against these activities, even within your villages. For your safety! We call upon you to evacuate your villages and move north of the Litani River.