Op--I totally understand.
The best "gift" my mother in law ever gave me was when she died. She was a horrible person. Of course, she avoided all responsibility so her 95 year old mother (who had been their personal piggy bank for 30 years) with dimentia became my responsibility. I remember my father-in-law telling me that grandma probably should have been in a nursing home 6 or 7 years ago, but if they did they she couldn't to continue to fund their lifestyle. Nice!
Grandma lived with us until she died, because her funds were almost gone when we took charge. I did send my father-in-law a demand letter for all the promisary notes he signed, still waiting... The inlaws didn't attend the funeral, we only heard from them to see about an inheritance.
My personal favorite was the first Christmas we had Grandma when my then 20 year old sister in law, cried because "grandma didn't give her a nice enough gift". Grandma lived 10 minutes from them at my house, and no one had visited her in 6 months (Grandma didn't travel so well). Seriously, grandma who couldn't take care of herself, which meant I did the shopping. Grandma gave her a $200 gc to a retailer, and she also got an addtional $100 from my husband and I. That was the last year ANYONE in my husband's family over 18 who wasn't a student got a gift. We would typically give $500-$750 in gifts to my father-in-law and the kids. We would receive $50 total in gift cards in return to the gas station. Two years ago the last person in my husband's family "aged" out and I no longer buy any gifts for his family.
I agree with others if you must do something, make a donation to a charity. Otherwise do nothing, or remind them they have grandkids and let each of them make a picture. If you spend the $$ you would have on some art supplies for your kids, I am sure they will have fun, and you can send a small sampling of the finished product to your in-laws as their gift. I think the funniest thing I ever saw was when one of my friends had her kids do a painting for her in-laws. She took a picture and included it in a card, that said Merry Christmas the artwork was to valuable/fragile to ship; but next time they visit they could take it home. The artwork is still on my friend's wall.
Keep in mind, I am jaded and no longer have to deal with my in-laws but I wouldn't spend money or time on people who suck. You can however make a donation to heiffer international and send a real message. I think you can still "buy a donkey", you get my meaning.