Wedding Invitation Etiquette
Invitations are always addressed to both members of a married couple. An invitation to an unmarried couple should be addressed with each name appearing on a separate line, the woman's name on the first line. No abbreviations or initials are used. If children are not receiving a separate invitation, their names may be written on a line below their parents' names on the inner envelope. If no inner envelope is used, their names must be written on the outer envelope.
For more information click on these links:
Crane's Etiquette Guide