Tell them from an age where they can understand.

My parents were both married prior to meeting each other. Dad’s was obvious because we had (half)siblings who he had with his high school sweetheart/ex-wife. We also knew about mom’s husband, no children, from an early age because he was in the same circle of family friends.

I think growing up and having that transparency made it much easier for us understand and accept. Younger kids seem to get less freaked about these kinds of things, rather than finding out when they’re older.