The following question is paraphrased from an email I received...
My 5-year-old daughter invited a friend to her birthday party, but the friend was still upset about an earlier conflict between them and said, "I don't want to go to your birthday party!" My daughter was clearly hurt by this rejection. How can I honor my daughter's feelings without invalidating the other girl's feelings?
Firstly, if you believe your daughter was hurt by the rejection, you're giving power to the other child's behavior, and you can't help either one of them feel authentically empowered from that perspective. So I would begin by reaching for empowering thoughts like "That behavior has no power of its own... We are all empowered from Within... External conditions have no power over us; it's our thoughts about conditions/behaviors that determine how we feel... No one can stop me from thinking a more empowering thought..." etc.
Once you feel centered in your own Power, you can begin to see things more clearly. For example, you might realize that your daughter's friend is simply trying to reconnect with her personal power using the only tools she can think of (i.e., the "dominator" tactics modeled by our culture). In other words, she doesn't want to hurt your daughter, she wants to feel powerful and doesn't know how else to connect with that feeling. Likewise, your daughter wants to make the same connection and, for the moment, believes that the rejection is causing the disconnection.
So they both want the same thing, and now the question is how can you best help them get it? As I said above, it doesn't help to join them in their disempowering thoughts (often called "empathy" or "validation"), but neither does it help to resist their disempowering beliefs and tell them nothing is wrong (i.e., invalidating).
A third way is available, and it's much more subtle. The part of you that wants to "DO something about it" will not be satisfied by this approach! Most of the "action" will take place on the inside, so it may seem like you're hardly doing anything at all.
The essence of this inner action is... making peace with What Is.. being able to look beneath the surface of their behavior/perceptions and know that All Is Well, in spite of appearances.
Another way to describe it is "seeing the Light at the end of the tunnel." Instead of seeing a problem, you see the solution unfolding — you expect them each to find their way and connect with their Center. They can feel that expectation, and that feeling is all they need to begin reconnecting.
This whole approach is encapsulated in Gandhi's famous advice to "BE the change" you want to see. If you want your child and her friend to KNOW their inherent powerfulness, regardless of anyone else's opinions or choices, you've got to KNOW YOUR OWN and then interact with them from that place of knowing.
Getting to that place of knowing is the inner work of parenting — and if you ask me, it's the most important work you'll ever do as a parent. :)