Do not burden the children with any of the anger or frustration you may be feeling.
Do not give too many details about why you have decided to separate or divorce.
Do not blame the other parent.
Do not make hasty promises or give children false hope of reconciliation.
Do not overwhelm children with too much information. Keep discussions about moving, changing schools, lifestyle changes, etc. for another time. Older children may bring these points up, but if these decisions have not yet been made, only speak about them in general terms.
Do not tell your children to keep the separation or divorce a secret, as they will need support from their friends and extended family. If you are not ready for others outside the immediate family to know, then this is not the time to tell the children either.