We used an egg donor for our DD2 (my good friend). We visit her twice a year and fom a really young age I used to say to my DD "we are going to vistit "K" and she helped mummy and daddy have a baby in mummies tummy". I left it at that and when she was around 4 she started asking her own questions.
I also have the Kate Bourne book (Sometimes it takes 3 to make a baby). I like the fact that this book has a section in the back for her to add her own story (like drawings, writings, photos etc). But I havent given this to her yet. I probably will over the next year.
I'm actually also in the position of having DD2 from our donor and DD3 was conceived naturally (with another on the way). Is anyone else in a similar situation. How do I deal with questions involving the different ways of "how I was made"??