If you choose to have this friend donate for you, assuming you establish roles that you are both happy with etc etc, think of the issue of the child knowing like this - you cannot keep it a secret forever... these things always have a way of slipping out. How do you think a 16 year old would react to finding out about this 'lie by omission'? An 8yr old?? How about a 2 year old... you can see that they younger they are, the better they take things like this in their stride... I plan to explain to my son as soon as he is old enough to understand, with age-appropriate terms, how it is that he just has a mummy. I don't want to be accused of lying when he gets older.