Five year old are easy to keep occupied. Go to the beach, have a paddle, and ice-cream and build sandcastles. I have never known a five year old find the beach boring, even on dull days!
If its raining go to the cinema, the amusesment arcades, the fun fair- maybe there's a pier show in the evening, or if it's fine a lovely walk by the sea?
There are usually lots of things going on that are free too: parks, museums but ifyou have money there may be history events on at nearby castles or stately homes.
I can't believe someone who was a teacher needs to know all this- seasides haven't changed that much I don't think.
I would avoid Butlins though, had my worst ever holiday (and the most expensive!) when my kids were little!