There are a bunch of websites that address your very issue with great meal plans.
It really isn't a complicated diet goal, just a lot of common sense. Things we should all be doing. Eliminate sugars and processed foods and restrict starches and salt. Incorporate low fat dairy (no hard cheese), fruits and vegetables, lean meats, poultry, fish, whole grains, nuts, beans. Drink only unsweetened beverages (no regular soda and juice only as a treat), tea, coffee, water, diet soda (if she can't live without soda but work on replacing it with something else). Getting the elderly to eat a balanced diet is not easy. Preparing the meal becomes a chore, cleaning up after the meal is work, their taste change, meds can interact with foods, the cost, they forget... Start by creating a list of meals and foods. Have your grandparents go through the list. What do they like, dislike, allergic to...Loves Chinese, hates fish, so-so on broccoli... Even if you prepare the best meals, if they don't like it, they won't eat it. Snacks are important for a diabetic to maintain glucose levels. Prepare and portion out snacks in bags like nuts, granola, tortilla chips, sliced apple and carrots... Soups are usually well received. You can make a pot of soup, portion it out in heat and serve containers (anything that goes straight from the freezer to the microwave to the table will help them. Go buy some new if you have to). A bowl of hardy bean soup, a handful of crackers and 1/2 an apple would be a good winter dinner. If you still have some leftover turkey, makes some TV dinners and pop them in the freezer. Turkey, 1/2 baked sweet potato, green vegetables done. Pay attention to portion sizes. You will be surprised just how little they eat. You can plan your home meals around grandma and grandpa too. Say you make a potroast for dinner. Portion up 2 meals for gm and gf. Pop them in the freezer. Do this all week with your dinners making a little extra and changing things up like the vegetable choice based on what they like. If they love green beans, keep a bag of frozen beans in the freezer and just plate their meals up with them if you are having something they don't like. At the end of the week, you have all their meals prepared for the following week and it took you just minutes to do it.
If you are doing the grocery shopping for them, you are in the driver's seat. You will keep foods out of the house that they shouldn't be eating.
I think you are doing an amazing thing. There is a huge need for this type of service and you could easily expand it as a community service or even a business.