For anyone who worried about the meta-achievement and wow gold for the Children, the achievements associated with the holiday haven't changed, but the quest lines in classic Azeroth have. If you're Alliance, the quest series starts with Children's Week, offered by Orphan Matron Nightingale in Stormwind; if you're Horde, it starts with Children's Week in Orgrimmar. Both new quest lines will see you surveying the changes to the local landscape in Cataclysm, in addition to meeting important lore figures and paying your respects to the dead. Afterwards, you'll go out for the traditional ice cream treat and then visit a local vendor to get kites to fly.
The quest lines end with the offer of a new noncombat pet in addition to the usual Piglet's Collar, Rat Cage, and more wow gold! It looks like the Outland quest series will also offer a new pet called Legs, though they don't appear to be any new quests there.