Soon I will be making a return trip through the Eurotunnel by car, each way arriving at the terminal at about dinner time. What options are there to eat dinner there, at both the Folkestone and Calais terminals?
At Folkestone there is a building with a Burger King and Starbucks at the waiting area between ticketing and passport control. After you pick up your ticket, stay to the left to go to this area. If you've already gone to immigration, there is nothing to do but wait for your train and eat in France.
At Calais you will find the same, Burger King and Starbucks, this time after both ticketing and passport control. At the roundabout immediately after UK customs take the first exit, to the car waiting area.
If Burger King isn't doing it for you, there are much better dining options landside at the Centre commercial Cité Europe in Calais, which is adjacent to the terminal. There used to be an entry directly ahead of the Eurotunnel ticket booths, but this appears to be closed. For the moment if you want to go here, try using exit 41 from the motorway.
Unless you understand "dinner" as "grab a bite", I would suggest to have dinner somewhere in the vincinity (maybe even after having gone through the tunnel). It will be better in any case, and prices will be better as well.