So the answer to the question is that for me, the answer changes depending on the situation. I say, I have 5 children. This always promotes a question, and I have the opportunity to educate someone new about SIDS, and share the story of my beautiful boy. I find that when I don't acknowledge my daughter I feel horrible afterwards, so this is not a solution for me. But at the same time I don't want to create an awkward moment with everyone I talk to if there is a way around it.
People see the necklace and ask questions.
They can usually figure out the meaning of the angel, and I imagine they would know the meaning if it was the only charm on the necklace. I also feel horrible if I don't include her in my answers. As far as reunions go, I bet that one of your classmates has experienced a similar loss. I really found a lot of neighbors, church acquaintances, etc. that also lost children in various stages of pregnancy. My school publishes a reference book for ex-students that shows where all of the graduates live, occupations, their phone numbers, spouses and children.