At that age, I just don't think there's much you can do to prepare them really. I've got a 15 month gap, and while I told DS that 'there's a baby in mummy's tummy' and he even knew where to point if we asked where the baby was, you could tell he just had no clue as to what it all meant in reality. He worked it out

. It can be a big adjustment for them, but they do adjust (and it's so much fun when they start interacting, totally worth it!)