If you are writing names in chinese, you only write on envelope, cos pple will automatically look at the chinese wording which has no "blank" for names.
If you are writing in english, you just write on outside of envelope and inside of the card on the english side.
In the card for family just write "Adrian Wong & Family" , same as outside on the envelope.. if it is in english. If in chiense, you only write outside the envelope the name of the head of household + lian2 jia4 which means whole family.
If you know the children but not so close to the parents, you still write the parents' names.