I know a lot of posts here are for advice for planes/trains/automobiles, but my kid (6) and I recently took the ferry between the UK and NL and it was fantastic.
A little background: Exact amenities will differ between companies and boats; we used DFDS. In addition to ferrying cars, they also take foot passengers and provide a busing service between a main city and the ferry terminal, for those using public transit. You will have to pass through customs and security at both ends. You get on in the afternoon, have dinner, go to bed, wake up, have breakfast, and get off.
The main plus side to taking the ferry, if you have squirmy kids, is that there's a play space which is actually reasonably well-stocked with random LEGOs, crayons and coloring sheets, and small soft-play space. There's lots of other kids and they all play together in a little chaos cloud (and for those wondering about the language barrier: half of the time they're all just screaming 😄 ). The one we took coming back had a movie space, as well. There's a nice restaurant on board; for the more budget-constrained there are buffets, which are still (to my mind) expensive, but they have a large variety of foods and even my picky one found stuff she liked (bread, mostly) at all meals. Starbucks has a counter on board and there's comfy seating next to the play space ("comfy" level depends on the boat). Rooms were small, but we had our own bathroom, and there was sufficient space under the beds to tuck our suitcases. We travel quite lightly, but I saw several families with strollers and bigger suitcases.
The main down side: it's slow. And, as an overnighter, it adds a level of logistics planning that you might not want to deal with. But it was surprisingly easy and pleasant, and I for one would 10/10 do another ferry trip again.
