We did a mini cruise to Amsterdam without the kids on a rare child free few days last year but both thought the kids would have enjoyed the ferry very much and may use the service to visit relatives in Holland in the future with them. I would recomend spending just a few pounds extra and booking an outside cabin rather than an inside. The inside cabins looked quite claustrophobic and had bunk beds rather than 2 singles. I would also avoid eating on the ship as this was quite expensive. it dosn't sail until about 9pm so you will have plenty of time to get something in Hull before you board. We took some crossaints and pastrys for the morning and took them up on deck and watched the ferry coming into Rotterdam. There is a small childrens entertainment place with a ball pool and place where they can do colouring, watch telly etc. There is also a cinema which was quite reasonably priced showing a couple of family films which would pass a couple of hours.