Bowleaze Cove Holiday Park in Weymouth. It's a caravan park but more upmarket than Haven. They have deluxe lodges with hot tubs but even the cheapest caravans are really nice. There's a spa, a great toddler pool with a slide and water play, soft play next to the bar, outdoor play areas, nice ice creams on the beach.
Loads to do nearby like aquarium, Monkey World, steam train, dinosaur museum over in Lyme Regis. We spent a rainy week there in October and never got bored!