This question can be viewed in many contexts. For me, symbols mean what the person intends for them to mean; for example, there have been several discussions here about this flag:
For many people here that symbol is a proud reminder of their southern heritage; others have used it as a symbol of racial hatred. Which is it? One might argue that it depends upon the intent of the person displaying it.
Another example:
To a German Jew, this symbol...
...the swastika, is a reminder of man's inhumanity to man, and the Nazi extermination of European Jews. On a trip to Korea I saw that symbol all over the place, on building after building, and it was marking a Buddhist pace of worship.
My point is, I seriously doubt if you have any reason to worry that this symbol...
will turn your daughter into a satanist or anything close. She has no intent of it meaning anything except "peace" does she?
My daughter has a jacket with a huge, ornate, sequined Peace Symbol on the back, and I assure you she is a righteous young woman.
This is not the hill you want to die on.

"God is excited to show you mercy. He rises to give you His compassion." (Isaiah 30:18)
"For surely, O LORD, you bless the righteous;
you surround them with your favor as with a shield." (Psalm 5:12)