Have you tried that in the tight quarters on an airplane? Because we didn't have much success helping kids on an airplane, there just wasn't enough room. Anything that required mom stayed in the backpack when we realized we just didn't have space. It was hard when my daughter was a toddler just getting her to suck a sippy cup during take off and landing. (Do bring something to suck during takeoff and landing. I woudn't recommend suckers because of the stick - chewy candy, something to drink).
I wouldn't do donuts either - I agree, too messy....you want non-squishable crumbs like graham crackers. (Ever have a kid spend an hour sitting on a fruit snack - not a pretty sight either).
However, if you feed them at the airport, instead of on the plane, you'll have more room and more options. Then you can do yogurt (or even cereal). Then, once they board, keep them occupied with the "brushables" - like goldfish crackers and dry cereal.