I am Indian and I have worn henna on my hands since I was a child. I loved everything about it from the smell to the designs minus how it looks when it is fading. In terms of meaning, it is not just for brides in wedding but in general for better health and prosperity. Men also wear henna in some communities. I just put on some cool designs for my son last week.

I don't think a non-Indian wearing Henna is cultural appropriation. Henna is worn in many middle-eastern cultures too.

Your daughter should have as much fun with it as she wants.