Most people with kids have some sort of a bedtime routine for them - or at least try to. It helps the kid with understanding the schedule and accepting that it is, indeed, time to go to sleep - and there is not much they can do to avoid it. While we are still working on convincing our children that they actually are supposed to be sleeping at night, we have at least established some sort of routine. After getting into pajamas, Anna gets either ten more minutes to play or one or two stories, depending on how late it is. When I lie down with her in bed, I ask her if she has any questions.
I've been doing this since long before she could actually comprehend what a question was, and over time her questions have gotten more complex and more serious. While she will often ask me questions about what we will do in the morning, or who is coming to our house for Shabbat, she tends to take the opportunity to ask me about more serious topics. She often broaches the subject of race, specifically why she is brown and I am tan. We talk about different angles, depending on what's on her mind. But I always reinforce the idea that she is not alone - I enumerate all the people we know who look like her rather than me. We talk about the many people in Israel who look like her.
Using G-d in the answer is important on the one hand, and confusing on the other. On the one hand it's a simple answer - G-d made us all a little bit different, but we are all people. On the other hand, it's adding another vague element to an already vague picture. Anna knows she's adopted - meaning that she knows the word. She understands that her skin color looks different, and she knows the words I've told her about growing in another lady's tummy. But beyond that, she is so young and is still forming opinions and ideas about the world and where she fits into it.