A friend of mine is 7 weeks ahead of me & had a little bump at 13wks so I pointed to her belly & told my little boy (2yrs at the time) that she had a baby in there. we were still keeping bubs #2 a secret but we took him home & told him that I had a baby in my belly & asked him where his baby was. He pointed proudly to his belly & said in there! This worked well because he was finding out his body parts & what they do.
I'm now 24wks & Seth blows raspberries on my belly & says that he's tickling baby. He puts his mouth on my belly and talks to the baby & now he puts his hand on my belly & tells me where the baby is kicking. It's so cute!
We also moved Seth into his own "special nemo bed" (a single bed with Nemo quilt) before we told him about the baby (a week after he turned 2) and have moved his cot to the baby's room. He now says that's baby's bed in baby's room. And his car seat was replaced by a booster seat just recently so his old car seat is baby's seat. It's a slow process but I too am worried how he'll handle having to share his mum & dad with someone else. He's doing really well & I'm less & less worried about the arrival of bubs.
Hope this is helpful!
CJ, SJ 3/03, AJ 11/05, PJ 4/08