When should I let my child know about the pregnancy?
The little amount of research done on sibling preparation suggests that children should be made aware of the pregnancy as soon as possible. They will more than likely have an 'inkling' that something is different anyway, and telling them may help any confusion they may feel.
Some people avoid letting their older child know about the pregnancy, until they are quite advanced (or possibly showing). This could stem from not wanting to let anyone know because of possible miscarriage, or waiting until test results suggest a healthy baby. It may also be a concern that the toddler, or child, will blurt it out to grandma!
The other side of this is that children know, understand and are able to deal with more than we give them credit for, and often overhear conversations and are aware, even if not explicitly told. This can apply to the joy of discovering a pregnancy, to the grief experienced due to a miscarriage (or deciding to terminate a pregnancy for abnormalities).