What it looks like, sounds like, feels like. Also discuss what they can do to help you. Perhaps assign them a job, depending on their age and ability. Our daughter, Hilary, will be nearly six when our next child is born. She is in charge of making sure I have a drink and bringing fresh washcloth.
Age appropriateness is also an important issue.
Never give them more information than they need or are asking for. It may only serve to confuse or overload them. Show them faces mommy may make during the birth Other things of Importance Your child will need to have a care provider of their own, not you. It is best if it is someone that they are close to and have had some preparation with. You and your partner should not have to deal with the child and probably will not have the energy to do so during labor.