I think your parents would love a nice picture of the cats playing with the toys with a note from you. But a thank you note from a cat? Come on, that’s just silly, we all know cats don’t do that grateful thing.
You people are no fun 
. Minx always sends thank you notes, she’s a properly brought up kitty. She prints very neatly and is a good speller. (I also have a rubber stamp of a paw print :o). The notes she sends are generally along the lines of what betenoir and gum were talking about - not overly cutesy (I think she did do a backwards “D” once, but she was just a kitten then, so I let it go).
I for one would love to get a thank you note from a cat.
But I would assume that the cat actually wrote it himself.
Some people are really good at it, like my friend who writes in a very teutonic style for her german shepard and like a hyperactive kid on crack for her jack russell terrier. But most pet letters do sound glurge-y. However, if your parents are into the sentimental glurge-y stuff, it will probably make them smile. Making parents smile is sometimes worth a small loss of dignity.
Like anything else on the sugary-sweet fluffy side:
If it’s written / handled well, it’s cute. If it’s not… well. 
There is a lady around here who is very active in the animal welfare community. She’s a lawyer and generally seems to have her head on straight. However, her hobby is writing letters from her cats. Occasionally, from her dead cats. Yes, her zombie cats have been endowed with literacy and returned from the grave to share their advice.
It takes all kinds.
I forgot to add: sometimes the letters from her dead cats are in French. She sends them to all manner of people–and animals, too; George Bush’s dog got one of these missives. She’s rather verging on batshit crazy rather than cutesy-wootsy, if you ask me, but it seems to be her only eccentricity.
George Bush’s dog got one? hehehehehe 
Do you know what it said?
I don’t know, but I’ve heard she’s trying to get a collection of the letters published.
I’d love to read them, hehehehe Thanks, burundi