I say nothing. Help out around the house when asked and in return I will provide food, shelter, and anything else that comes up. I tried the allowance thing and it didn't work for me. They started off well enough, but got lazy fast and still expected to get paid. Now I just tell them to do something and they have it done by the time I'm home from work.
I think that more than worrying about how much, you should think about if this is going to be an opportunity to teach them something usefull: responsibilities, obligations, limits, money management. I would talk to a child psychology and ask for guidance. You want to do good on this one.
I don't get any allowance no matter what I do. I think the best thing is to just not give it to them, because if they don't even do anything they are going to become brats and they aren't going to learn that in the real world, you don't get money for sitting and watching TV all day. If they ask for money every now and then, it's OK. But definitley no allowance for a 6 year old. What will they do with it.? Save your money.!