I know at McDonald's type places lots of people have the "you can't play on the playground or have your happy meal prize until you eat" rule. But I learned long ago with my nephews, if they could hit the playground first, we could eat in peace. Then when they were done playing, they were starving, and ate everything. So when traveling, we would stop at McDonald's or somewhere with a playground for lunch. Let one adult take the kids straight to the playground to run off steam, while the other adult got food for just the adults. Both adults eat/watch the kids play.
After the adults ate (this does work better with two adults), one adult would get back in line and get the kid's food "to go," and the other would take them to the bathroom, wash hands, etc. We'd get back on the road, and let the kids eat their food in the car. We ended up having a shorter stop this way, and the kids blew off some steam.
My friend has done this solo with her kids. She just takes them directly to the playground, then when done playing, she gets everyone back in the car, and goes through the drivethrough to get everyone's food, to eat while driving.