Hello Roger,
I cannot recommend a site, but do know Abersoch well, my advice would be to get as close to the the centre of Abersoch as you can. The beach is fabulous, very flat and children will love it. There are many wealthy people with places and they have their boats towed out across the beach, by Farm Tractors, to get to water deep enough to float.
There are plenty of options for you to consider have a look at the following site, which shows them all, and has lots of reviews by previous clients.
http://www.ukcampsite.co.uk/sites/results3.asp?search=ABERSOCH&by=town
Good luck and enjoy, the weather should be good, as it has a micro climat.
Best regards,
George