For years we took our kids to Mumbles on the South Wales coast, just along from Swansea. It's a fair drive from you, but we all thoroughly enjoyed it. Mumbles is next to the Gower, which is a fantastic peninsular with the most amazing beaches. Caswell Bay was a firm favourite as it has easy access, car parking, toilets, a shop, and most importantly, a stream which runs across the beach which can be dug in and dammed all day long! There are plenty of other beaches to enjoy though, Port Eynon, Horton, Rhossilli (with a shipwreck), Oxwich Bay (we always had tea in the Oxwich Bay Hotel when we went there), and some brilliant beaches with less easy access, but worth the walk, Pobbles, Three Cliffs Bay (amazing). Plenty of stuff to do in Swansea and surrounding area if the weather isn't brilliant (bound to happen one or two days). Kids tend to enjoy the simple stuff, a beach with a bucket and spade, an ice cream, maybe a trip to the amusement arcade in the evening for a go on the 2p falls, and they're happy!