I have been to both and would recommend both. Lac des Reves is much quieter and you need a car to get to anything. La Carabasse has shops/restaurants/beach etc within walking distance, but it will be quiet that early in the season. We were in Vias once (on a canal boat) at the end of May...the weather was lovely, though the sea was freezing. Some of the bars and restaurants were open but not all. Luna Park doesn't open until July.
I believe the kid clubs run all season, though with your dc's you would have to stay with them the whole time anyway - you can't leave them until they turn 5. The pool at Lac Des Reves is probably better for smaller dcs than the complex at La Carabasse.
I would recommend getting the best mobile you can afford. We had a 2 bed Esprit at Lac des Reves and 2 bed Excellence at la Carabasse. There was world of difference between the 2 in terms of space and comfort. We even got a washing machine this year! The Esprit was fine by the way - no complaints - but it is lovely to have a bit more space.
There is loads to do in the area. Beziers and Montpellier are definitely worth visiting. There is an aquarium at the Le Grau de Roi. Aigues Mortes is lovely. Meze is great for Oysters and fresh seafood.