In Fable 3 for the Xbox 360, the Brightwall Demon Door wants a child brought to it before it will open, however the only child in the game that you can lead there is one of your own.
You can either have your own child through the normal means, which would require the Family and Lover Expression Pack, buying a house, marrying someone and having a child that way. Alternatively you can adopt a child from the Bowerstone Orphanage after you complete the Bowerstone Resistance quest.
Whichever method you choose, simply lead your child to the door to open it. Inside you’ll find three Summon Creature Potions, and 25 Guild Seals.