Addressing an envelope to a family can be as simple as writing "The Smith Family" as the address's top line. However, the rules get a bit tricky when you must stick to proper etiquette guidelines such as when you send the family formal wedding or graduation invitations. How you address an envelope to a family depends on several factors, including the contents of the envelope and the children's ages.
When you're sending a casual letter or card to family and friends you know well, you don't have to worry too much about proper etiquette. The way you address the envelope should fall in line with the contents -- a funny holiday card, for example, doesn't require formal titles on the envelope. Instead, use the first names of the parents followed by their last name, then add the children's first names on the line below. The first line might read "Joe and Jane Smith," and the second line might say "Jack and Joan." Instead of listing the children, writing "and family" on the second line is acceptable.
Formal Envelopes With Inner Envelopes
Many formal pieces have two envelopes: one for the delivery address and an inner envelope that holds the invitation or other formal announcement. In these cases, address the two envelopes differently. In most cases, the outer envelope should be addressed only to the parents using formal titles and full names instead of nicknames. The inner envelope is where you list nicknames, titles such as "Uncle," and the children's names in the order of their ages. One exception is for children 18 and older who still live with their parents -- it's proper to either send them their own separate invitation or include their names on the parents' envelope.
Formal Envelopes With One Envelope
When your formal piece has only one envelope, include children's names only if they are invited to the event or are otherwise included, such as when you're announcing your graduation but not inviting people to the ceremony. If the children's names aren't on the envelope, it implies they aren't invited to the event. For example, write the parents' names on the first line, complete with proper titles. On the second line, list the children's names in birth order. No last name is necessary on the line with the children's names.
Different Last Names
While "Mr. and Mrs. Joe Smith" is acceptable when the wife uses her husband's last name, respect the use of her maiden name if that's what she goes by. In that case, you typically list the woman's name first, or list the names alphabetically for same-sex couples. You might address a casual letter as "Jane Jones and Joe Smith," with "and Family" or the kids' names on the second line.
For a formal envelope, use proper titles such as "Mrs." or "Dr." List the woman's name first. For same-sex couples with different last names, use proper titles and list the adults' names alphabetically on the first line. Only list the children's names as part of the address if there is no inner envelope.