D.K.
My daughter has brown patch on her neck below her ear. It actually is almost the shape of a butterfly. It basically looks like mud on her neck! A lot of kids asked what it was, she proudly spoke up and said "it is an angel kiss!!!", she actually has always worried her brother has no birthmark, so it is how you approach it. She asks everyone if they have an angel kiss and is very proud she has one. I have one on the back of my leg, hated wearing shorts, but now with my daughter really excited about angels kissing her that she reminds me it is nothing!
Don't set her up for worrying about what someone else says, it will come to a point a child will ask, most young kids are curious not malicious. Have her wear it proudly that it makes her unique. If she wants bangs then so be it but don't let her think it is to cover it up. Talk to her about it and explain what birthmarks are and have her be aware and be able to answer for herself without letting her know there could be ridcule. If it doesn't bother her, don't let it bother you. Kids are amazing in what they are accepting of. One of my best friends I grew up with had a birthmark, dark purple on the side of her face. I remember thinking how pretty she was, she had the bluest eyes, dark long hair and I looked past it and never even really paid attention to it. She was a great outgoing girl too! :)
As she gets older she may want to discuss removal, after her skin changes as she grows it may actually move or change. I know I have a scar I had on my cheek as a child, it is now below my chin line.