I can recommend Silver Sands beach near Aberdour. I also went there for weekend trips. It is easily accessible from train after Edinburgh. Good clean water, toilets, grass to sit on, bring bucket and spade.
http://www.flickr.com/photos/pdonald...7602320956842/
Video results of Aberdour beach
I hope that link works
There is also a caravan park called Pettycur with beautiful views across the Forth estuary.PettyCur caravan website
That would be self catering, but theres hotels and BB nearby there too.