Thanks for the recommendations. I typically prefer to stay in small B&Bs since they are all different and it's the only time of the year I get to have a full English breakfast. I'm looking into the places you mentioned above.
I wouldn't rule out Youth Hostels - some of them are pretty splendid (
St Brievals is an old castle, and you sleep on a dungeon with medieval graffiti, for example). There are lots of other ones that are equally splendid - and wiith more room for a nine-year old to run about or explore too.
https://www.yha.org.uk/hostel/yha-st-briavels-castleThe Pembrokshire coastal path could be another option, and gives you some seaside, with some really nice beaches too.
