Held from the beginning of May, and lasting a week, it is a time for the heroes of both factions to give back to the innocent victims of war... the orphans.
The original version of the quest started from the orphanage in Orgrimmar or Stormwind City, depending on your faction, and gave you a whistle that summoned an Orc or Human orphan. The orphan would behave like a non-combatant and he would give you quests, requiring you to travel all over Azeroth. Open completing all 7 quests, the player is rewarded with 1 of 3 non-combatant pets to keep.
Since the quest is repeatable every year, but only once per year, you can eventually have all the pets.
Children's Week is also featured in Outland for players with access to the Burning Crusade Expansion. The orphanage is in Shattrath City and is very similar to the original quest, making you take your orphan Blood Elf or Dreanei on a trip over Outland and some of the new areas added in Azeroth from the expansion. Once again, the reward is one of 3 pets.