Should I tell my child their twin died during the birth?

I think it would be a good thing for them to know, but age and maturity level of the child are important factors to consider when thinking about telling them.

You should tell them when there young cause it will hurt them more when they get older if they find out then but don't cover this up because most likely they will find out and if the parents don't tell them it will also hurt them just be there support them and help them.

When the child is old enough and you are emotionally steady to take on the child's questions and grief.

Same as I said to another question. Well, it wasn't their older sibling because it didn't live. I'm so sorry for you, sweetie, and you should tell your child, "Excuse me, but can we have a talk? You didn't do anything wrong, don't worry. It’s about my first child." Then you can say, gently, "You see, I had another child before you were born. But unfortunately, he/she died, so you didn't get your older sibling." Give them a hug, answer any questions, and be supportive. I hope that helps, and I hope you'll find joy despite your child's death, sweetheart.

Yes, but wait until they are old enough to process that information. It may help them feel better about some things.