I watched how my brother and sister-in-law explained to my little three year old niece that she was going to be a big sister. They read to her from books, they had her touch mommy's tummy and tell little sister was in there, and they told her what a big girl she was and how now she was going to be a big sister and what fun that was going to be. Always talking to children about the news is the best way to let them know what's happening.