When I was pregnant, my partner and I told the kids together. They had to know to be more careful with me because their little brother was inside, and they loved talking to him and singing to him before he was born.
Try to make it sound like a positive thing - like they will have a new friend to play with, or make them feel important by saying they'll have added responsibility to look after their new sibling. Tell them early so that they have several months to prepare, but not so early that they grow impatient. When you start showing is probably a good idea.
My husband and I told her daughter she was going to have a baby brother together. We just told her that matter of factly. She was only two and a half at the time so we waited until I was showing to tell her because nine months to a child that age is practically forever. She sat in on my appointments with my midwife and "helped" and was generally pretty excited about it.
As gentle as possible....Say that in a few months ''something wonderful is going to happen to the family'', that you all have been given a miracle of life.
If it is told in a exciting way, because it is a new experience...that you all will be very happy....
Say that you can go out and buy a few little things for the new baby.If you are asked 'how' the baby got into Mummy's tummy....unless the child is over 12, just say it is one of life's biggest joys...that happen at times.
If the child is 12 and over, your on your own! lol that's just another joy for you as parents to get through :) (Use Google for ways to express it, that come close to your lifestyle.) but be very careful....Some children can find that truth, a little hard to except.