My dad, sister and I went to Ireland on the overnight ferry and it was brilliant! I was about 7 or 8 and my sister 4, so older than your children but we loved it. My mum had got us mini rucksacks with our pyjamas and toothbrushes in to take from the car and the cabin and bunks were loads of fun. We played in the play area and sat with dad while he had a Guinness then all went to our cosy cabin. It is one of our fondest memories: highly recommended!