Given the boy can't leave the country without the help of Make A Wish (£2,500 for three days travel insurance to go to eurodisney, ffs) and actually we have to go through a load of admin to leave the county overnight, I'm looking at buying a tent. If its going to be to somewhere uninspiring he may as well get a kick out of unusual accommodation options.
Pembrokeshire is very beautiful and there are some excellent campsites near Freshwater west, which is a fantastic beach for kids. Alternatively rent a house inland, near Narberth. It isn't very big, so getting to any part of the coast usually only takes 30 mins in a car and you have Morriston hospital in Swansea about 45mins away in the worse case scenario. It's also pretty quiet there, as in nowhere near as busy as many coastal resorts. It does suffer from a lack of great pubs, but there are a few.