I remember my mum being rushed out of house and me (aged 4) going to stay with neighbour, was because mum was having miscarriage, which I only learned years later. I'm sure my older siblings were only told once each new baby arrived safely. I told my daughter quite early on so she wouldn't be frightened when I was being sick so often and we'd read stories about new babies a lot to prepare her, my husband was working away from home a lot at the time so I was on own with toddler a lot. We bought a present for her to give the baby when it arrived and also one for the baby to give her, a present she treasured for many years as was from her little brother. I think most people nowadays do tell children a new baby is on the way but I think it's wise to wait till 16 weeks or even 20 if it's a mum who's had early miscarriages in the past. Thing is toddlers overhear a lot more than we think they do and other adults will casually comment to the mum about the new baby arriving, which I don't think was so commonly mentioned back in the 1950s, so they'd be very confused if not in on the happy news. I think is something that needs careful handling with gradual sharing of where the baby is as mum's body gets bigger. My daughter used to speak and sing to her new baby long before he arrived and was wonderful about the new addition once he was here.