Although my children have not started school yet, I hear that it is a common project to assign the research and design of the child's family tree. I've also heard that it can be a little traumatic for a child to "want" to do a family tree that includes the birth family members about which she has been told (or has met) but to "not want" to do it if others at her school are not aware that she is adopted. If she is shy or otherwise not yet comfortable with her adoption, the thought of having to field questions from curious classmates might be enough to prevent her from doing so.
Because one of the benefits of creating a lifebook for your child is the open communication about adoption that ensues, including an adoption and birth family tree in the lifebook is a good visual representation for the child just how many people were involved in making her who she is today and it might help make that future school project a little easier to tackle!
I have been asked twice before if I had any ideas for designing an adoption family tree. My design always starts with a bare tree die cut but after that, I have lots of varying ideas. Leaf die cuts or punchies could be added to the base tree for decoration. One idea would be to use two separate kinds of leaves (maple and oak?) and write the names and relationship of adopted family members on one style and birth family members on the other. You could also do the captions in two different colors...blue pen for adopted family and red pen for biological family, for example. You could also include a poem on the page, perhaps the "Legacy of an Adopted Child" about how a child is the product of two different kinds of love.
I can give you a visual of what I was thinking. I just non-creatively arranged some things on a page today (didn't even glue them down!) and then snapped a photo. Sorry that it isn't "finished" but hopefully it serves some purpose. (If anyone is willing to share their pages, please don't be shy!)
My sample (names are fictitious): http://www.scrapandtell.com/images/FamilyTreeSample.jpg
Legacy of an Adopted Child poem: http://www.scrapandtell.com/searchid.asp?productid=1939