When you think about it, the offended parents not only chose to be offended, but chose to "traumatize" their child by taking away the snowflake. Chose to. And I won't say they were right or wrong to do so; their child, their business. I don't think that aspect of it is on the OP, and I wonder how anyone could just be expected to know that the gift would be offensive to this one child's parents.
This is a loaded issue, and after reading through all of these posts my own opinion on the matter isn't set. I think that, for the parents, it's a case of "It's not what you do; it's how you do it." They weren't wrong to express that they were offended, but I think someone else who posted came up with a much nicer way they could have said it. People react better to "In the future, please don't" than to "How dare you".
I remember that my second grade teacher was also Jewish, and I didn't know this until our classroom party (which we still had back when I was in school; I don't know how things are done now). I wished her a Merry Christmas, and she smiled and said, "Thank you, but I'm Jewish. I celebrate Chanukah." And she went on to tell the class a bit about Chanukah. Two of my classmates, I found out, were also Jewish, and they chimed in with their family experiences, sort of like show-and-tell. It was a lot of fun to learn about what people do differently!