R.M.
Wow...what a heartwarming "problem" to have!!! I can tell you from personal experience that what your parents want more than anything in the world is just to continue the warm and loving relationship with you and your family that they have had over the past months and years. I don't know how far you have moved away from them but I hope you are still close enough -geographically speaking- to let them continue to be a presence in your lives.
Our middle daughter came home a few months before she gave birth to her son who is now 6 months old. She lived with us until her son was 12 weeks old and then moved half way across the country to try and work things out with her sons Father. I cannot begin to tell you how my heart ached as she pulled out of the driveway with the precious little boy in the car!!! It is so unbelievably hard to go from seeing and loving on your grandbaby every single day to only being able to talk to him on a video call on your computer!!
As to what you can do to tell them thank you, they know that you are in a tight financial situation and they probably would be unhappy with you if you spent money that you don't have to buy something for them!!! I would think that the best thing you can do is just verbally let them know, how very much you appreciate their love and sacrifice for your family!! Let the children continue to have lots of contact...let them make a "thank you card" to send to their grandparents...get something that they can make handprints with and write little notes around the handprints that say how much they love their grandparents and how much they miss seeing them everyday.
You must already have a fabulous relationship with your parents...and they with you...that is the biggest "Thank you" that all of you could possibly have!!!
If you are close enough, how about setting up a weekly, bimonthly...whatever works for you...an extended family night and have them over to the house for dinner? Do everything you can to continue to involve them in your life and let them have lots and lots of contacts with those precious grandchildren of theirs!!!