How old are your kiddos?
When my DD was around 5 - I had made some up that were sort of preprinted - yet she could fill-in-the blank, and then DD put her own artwork on the front. Sorry - that was 10 years ago, so I don't remember exactly what they said.
When DD started getting older, she wrote the whole note herself. My advice - they can be short, do not edit them (this isn't homework afterall). They do need to be sincere! One of my favorites that DD wrote when she was about 8 or 9 - to her grandparents was : Thank you so much for the video chair. If I had known it would be so nice, I would have asked for one years ago.
When DD started getting gift cards for presents, she needed to complete the TY's before she could go spend the money!
Another year, when DD was in 4th grade, she took all her birthday money and we found a rolltop desk that she really wanted on ebay. We took a picture of her with the desk, and used that as the front of the Thank you note. The Thank you was more of a "Thanks for the birthday money, I used it to help buy my new desk!
This year (DD is now 15), she had all her TY's written by Sunday night, and they were in the mail on Monday. Don't know what she wrote - she totally sealed them up and addressed them.
Maybe you could help with the "format": i.e. each TY should have a:
Dear So&So:
Thank you for the Target giftcard! I am going to use it to by some games for my Wii. I can't wait to go shopping! Love, Frank.
Really - that is all that is needed in a thank you note. It doesn't need to be an essay.
Good Luck!