How do you address an envelope to a gay couple with a child?

Typically, you should ask them what they want, but that's not always practical. So as a married gay man with children, I suggest you would address it in one of the following ways:

If same last name:

John & Richard Smith & family

John & Richard Smith

Bobby and Susan

The Smith Family

If different last names

John Smith & Richard Jones

Bobby and Susan

Mr. John Smith & Mr. Richard Jones

Bobby and Susan

John Smith & Richard Jones & Family

Notes:

• Never use "Mr & Mr" unless you know the couple is okay with that.
  
• Never use "John Smith & Richard." If you don't know the last name of one of the spouses, then find out what it is. If you absolutely can't find out what it is, it might be best to address it only to one spouse & family. I have also seen just first names used, and this usually doesn't cause a problem.
  
• Never use "Messrs." unless you are over 90 years old and you want to convey the message that you think of the couple as confirmed bachelor brothers.