Sassy - I have 2 points on not telling.
The first is that we should tell to show that we are not ashamed, that this is no dirty secret, that this child is never to be hushed over. That this child was yearned for, so much so that no barrier was considered to high. That despite everything, the family searched and searched and finally found this child. Then found teams of people to help make this child a reality. Probably 20 people all working and studying, researching and fighting to create this child. When I consider it, I almost feel it should be yelled from the rooftops!
To know that you were born of that amount of love and dedication can only, in the long run and after much comunication and support, be a positive strong link with those around you.
ANd I believe that if a human being can accept and understand that yes, back on day one of their creation, a woman said to their parents 'here, here is a gift of an egg. You will need what is in it to get your dream' then they will achieve a huge level of calm happiness.
Sadly it is the human condition to second guess, to suspect, to almost assume and search out a negtive motive. But imagine if this could be got through, and the only conclusion possible was that someone wanted to be kind, and therefore you were born. It's all good. All positive.
Secondly, should a child ever find out or put two and two together, or some relative tell them for whatever reason, or should that child need a kidney and you decide to contact a seemingly random stranger to see if they will or can help...then the truth may out. By not sharing it from the begining it might seem like the opposite of the above is true. The child/adult may feel they are the guilty secret, odd, born of a strange and different make up. So untrue! So not how it is, not how you planned it, not how it's meant to be. Why make an issue of donation by denying it. Why not embrace it as one of the hoops you jumped through to get a child.
If you opt to go for the Spansh donation, I would still think long and hard about your motives for deciding not to tell the child. You may decide this is right for you and the chld and only time will tell and you may be proven right. However I would urge you to consider the possibilty of telling the child, although I haven't really thought through the process of doing this and the child-adult having no route to findng their donor. I am not sure how that could pan out.
It is a minefield. SO many things are. We cannot control and steer every ship through every sea. Ride your wave and see where it takes you, and I for one wish you all the best and safe journey.