La Manga is not on the Costa Blanca, it is on the Costa Calida.
I have only been to La Viñuela once, but from what I can remember it is rural and the main attraction was the lake. To be honest, I wouldn't have chosen the place as a winter destination for myself, and certainly not if I had 2 young children with me. That's not to say the place isn't picturesque, but personally, I think the whole place could be enjoyed in one afternoon.
To help myself and others in recommending a destination, perhaps you can tell us what did you and your children find so enjoyable?
Even though La Manga is relatively quite over Christmas, I can't see it being any more quite than La Viñuela. My main concern would be the weather.
La Manga is a peninsular, it is windy at the best times of year.