Havent done Cornwall for a few years now (since having ds) as drive from Scotland a bit much. Been to North Devon though a couple of times and can recommend Ruda.
Mainly for the swimming pool which has a large shallow end with some slides which ds loved. It also has a lazy river which he loved too. Small cove type beach with rock pools which we spent a lot of time on and also up the sand dunes.
Never did evening entertainment as we/ds prefers to sit outside in nice weather and drink guiness and cheap wine kick a ball about (hate the character shows!)
We went out of season as ds wasnt at school so got reasonable deals on the beachside apartments one year and Clovelly lodge another (the lodge was fab with 3 bedrooms, ensuite shower, bathroom, washing machine, balcony, excellent kitchen etc).
Weather was fab both times so never had the problem of looking for indoor activities.
Visted a couple of parks close by, one that sticks in my mind is Watermouth Castle which was small and relaxing and perfect for young kids - 1/2 dozen traditional small rides like teacups & aeroplanes, gnome land, crazy golf, maze, fab ice-cream. (and you could get a return visit another day for ~£1.50 a head) - everything is outdoors so you really need the weather.