A great source of sentiments for festive cards are Christmas songs. You'll find some inspiration for greetings for all the family. Here are some ideas and you'll find many more online at a lyric site such as Metro Lyrics:
'Tis the season to be jolly. Tra la la la la, la la la la
... and so this is Christmas, what have we done? Another year over and a new one just begun
Have yourself a merry little Christmas, let your heart be light. From now on our troubles will be out of sight
Merry Christmas baby
We wish you a Merry Christmas and a Happy New Year
Last Christmas I gave you my heart, the very next day you took it away. This year, to save me from tears, I'll give it to someone special
I'll be home for Christmas