On entry to the area to board the euro tunnel there is a doggy play park that is a vast area to allow the dogs the opportunity to let off some steam before getting back into the car and more importantly to go to the toilet. There are also lots of bins there to dispose of anything you need. There are benched so that you can also have something to eat and drink (there are places you can buy these from if you need to but take dog treats etc with you)
It is a free service on offer and wouldn't hesitate to use this again