I have heard of adoptees doing the tree with the birth family as the roots and the adoptive family as the branches and the leaves.
Grace attends a Catholic school that goes from Nursery to 8th grades. By the time she gets to the fifth grade project, I think the fact that she was adopted will be "old news". This year in nursery school, I did a presentation on the Lunar New Year. We talked about Grace being born in Korea, but didn't mention adoption. I hope each year to repeat this, but add more as they get older. Also, it will be based on Grace's wants and needs. Although there are few asians in this school, we like the fact that she will be with the same small group of kids until 8th grade. Like I said, it will be old hat by the time she gets to 5th grade. At the rate things have gone for Grace, the other kids will be jealous that she has a special tree while all the others have the same-old-same-old kind of tree!
Why do kids need to focus on family heritage? I think it's because as teens they will be asking the question, Who am I? Knowing your heritage is part of that. For Grace, her birth family and her adoptive family is both who she is. I could see her putting her fm as the trunk of the tree. Matt just did a science project on Mt. Etna. From a science perspective, he learned about volcanoes. Also, he talked about how his Great grandfather was born in Sicily near Mt. Etna and brought in lava my father had brought back from Mt. Etna. It was our choice to add the Italian heritage to the project, but I can really see how it added to his self esteem. Of course, it helped that he got first prize!
I think the biggest problem adoptive families have with the family tree project is when the teacher doesn't give them any direction. Parents are left to wonder how to do this. But I have heard of many making it into a positive thing.