It would maybe be easier to you wrote the card to the last name of the family so that it includes everyone and there's no confusion for example, To The Smith Family Happy Thanksgiving. However, if you are really set on writing all the names separately then no you do not need to include your daughter's name or your son in law's name since you already sent them another card.
Wow! First, they should just be glad you're nice enough to send a card on Thanksgiving. Secondly, it must be nice to be able to AFFORD cards for holidays! Wow! I am out of a full-time job. My kids will be lucky to get a few presents let alone anyone I know get a card for holidays!