Telling other children you're pregnant.

When did you ladies decide when to tell your children/ step children that you were expecting? My step daughter is desperate for us to have a baby and keeps asking questions.  I sometimes struggle to hide my sickness (how can anyone puke quietly lol).  I'm nearly 10 weeks and we've had a scan that has shown the baby's heartbeat and the doctor said that was a good sign so we don't know whether to tell them now or wait until 12 weeks.  Kids know us better than we realise and I think they can feel that something is definitely up