I want to take my niece to a concert without her parents
Are your niece's parents okay with her going alone to the concert with you?
I've already asked her parents but they don't understand and don't want her to go.
If your niece's parents don't want her to go to the concert without them, then you planning on taking her gets right in the middle of their relationship. Keep out of the business between your niece and her parents and drop the concert plans.
I'm taking her anyway.
That is highly inappropriate of you. Respect your niece's parents' wishes.