DD1 is 3.3 and her sister is a month old. Initially we wanted to wait until 20 weeks, so we knew everything was ok. Then we decided to move her into her new room, so that we could introduce the idea that she had a 'big girls bedroom' and then suggest that as the baby's bedroom was empty we would have another baby to sleep in there. So we told her when I was 7 months.
Toddlers are amazingly wrapped up in their own little world - she came to several MW appointments (although not scans), and accepted that the 'nurse' was just checking Mummy's tummy/arm/whatever wasn't poorly. Several adults asked her 'are you excited about the baby?' - and because she didn't have a clue, she let it go right over her head. Had one of her friends said something, I think she would have paid attention to them, and it was a possibility as several were getting little brothers or sisters and they were starting to comment on my tummy, so in the end we got in before they did. She still found 2 months the longest wait ever, despite having her birthday/Christmas/starting pre school to be excited about as well.