I have 9, 7, 6, and 2 year old girls. My older girls waited outside the bathroom while I took the test, and when I came out they were all asking what it said. When I told them it was positive they started jumping up and down cheering LOL! It was really cute.
I started telling my 2 year old about the baby in mommy's belly when I began showing so that she could have something to visualize change with. Sometimes I ask her where the baby is, what is the baby's name ect. She will answer, but she doesn't really understand, and she wont until baby comes home.