Actually it's not a good idea to translate it. Japanese mail carriers can read envelopes written in English format, and a bizarre mix of the Japanese and English styles would make your envelope look worse. Rest assured that you can always write "The Tanaka Family" or "Mr. Taro Tanaka", using English alphabet, on the first line.
(EDIT: By the way, did you know that, by writing the name on the first line, you are already violating "the Japanese rule"? On Japanese-style envelopes/postcards, the address comes first and the name comes last. That's another reason why you should stick to one particular format.)
You can stop reading this here, but in case you're interested, the polite and orthodox way to write this in Japanese would be:
Tanaka-ke go-ichidō sama
(lit. Tanaka + family + (honorific prefix) + everyone + (honoric suffix)
Here 家 (け,
-ke) is a suffix that follows a family name and refers to the family itself. A family name followed by さん (
-san) refers to an individual person in the family, like English "Mr."
But honestly speaking, writing something like this using English characters looks even funny to me. If you really want to write Japanese words using English characters, IMHO it's sufficient if you wrote:
Tanaka-ke no minasama
(lit. Tanaka + family + 's + everyone + (honorific suffix))