Old dogs... new tricks?

I would be very grateful for some advice on my 11-year-old rescue Collie cross Labrador, Rosie. We have had Rosie since she was 18 months old and she has always shown aggressive tendencies towards children. As Rosie is a nervous dog and has shown active defence reflexes (growling) previously in these situations, we have managed the issue by keeping her separate from any visiting children. However our circumstances have changed, as I am now thirteen weeks pregnant. Is it possible at this late stage in Rosie's life to improve her reaction towards children? Any advice that you could provide would be gratefully received, as we would never want to consider the option of rehoming Rosie.
Kate Bailey, by email