To access this demon door you'll need to bring him your own child and since you can only hold hands with one of your own you'll need to have unprotected sex with your wife which then she will give you a infant child so you'll have to wait and return when its just a bit older then bring him/her to the door (you can adopt one at the Bowerstone orphanage) and you will get a treasure chest which gives you 25 Guild Seals and 3 "Summon Creature" potions.