I have stayed here, a Haven park called Hafan y Mor.
http://www.havenholidays.com/Parks/wales/hafan ymor/
It is in North Wales, it is a large park (but not overcrowded)
and has loads of facilities. There are caravans AND chalets.
They also offer half board if you want that.
There is a large swiming complex, and a number of buildings with evening entertainment which cater for kids and adults.
It is in a beautiful area of North Wales. It is on the Llyn Peninsula and near the Snowdon National Park for those that may want to go off site.
Why not send away for a brochure from Haven, Pontins etc (or pick one up from a travel agent).
btw These places are not cheap in August so I think you will find it costs more than you expect.