Took the Brittany Ferries route some years back when the kids were smaller. It was a relatively bumpy sea, and my son was sick. OK, we had spare clothes, so changed him. He was sick again. And again. Of course, you’re not allowed to get to your car to get fresh clothes, even if you, your wife and your son are standing there, puke-splattered, and generally very miserable.
My abiding memory of that 4 hour crossing was spending most of it in the toilets trying to wash his least badly splattered clothes and dry them with the hot air blower. After 45 minutes of constant blowing the dryer packed in, at which point the room was in any case at about 30 degrees and stinking of vomit.
Never again. The next time we went to Brittany we just used the tunnel and drove an extra 3 hours during the night.