Well, maybe you eat some of the cs meals they love with them, and other times you split up for meals. I have had this same problem when travelling with family. Generally dh's aunt (who is elderly) and my own parents enjoy a sit down meal once a day. We about went broke travelling with them! In the end, its tough to please all people esp in a broad age range and its probably easiest to split up for a few meals.
If they don't want to do that you might tell them "sorry, we'd love to but no can do" and see if they would like to treat you to those extra meals. Or you could book places where you can share meals (that would be non buffet). I have a 13 yo dd who is a very small eater. We have never been to a non buffet restaurant in WDW where she can't order a kids meal or share with me or just order a side salad. That helps. Also ordering water only saves around $10 per meal. Breakfast buffets are cheaper than dinner. So there are ways to eat TS that is cheaper. I am not a fan of the
DDP, its just too much food for my 13 yo. So for us, if we eat TS from here on out, we'll do breakfast for the buffets, then share meals, order dd a kids meal and drink water to have a sit down experience for a much cheaper price tag.
You could also do some of the really good cs options. Wolfgang Puck Express is like a sit down meal. Its pricier than other cs, but you can share, there is no tip and its excellent. Eat there a couple of times as a happy medium!
I'll also recommend the Plaza Inn. Sandwiches, soups, salads, burgers. Its a sit down, but very reasonable by WDW standards. The Trails End Buffet is another cheap option and its a short boat ride from the WL.