hi there, S essex... not sure there are very many good beaches... they're all stony! well certainly southend, chalkwell and westcliff beaches are, the other direction southchurch, thorpe bay and shoebury arent much better, i guess shoebury would be the best of the lot... but in my experience they're all pretty grim!
southend has the odd sandy bit of beach (man made) and my ds loves it (although it isnt the best resort, I live there so it is walking distance) it depends what you are looking for. Ds loves riding on teh train up the pier as well. There are lots of little cafes.
Like cushion said shoebury (east beach) is ok and has parking. Not sure of other beaches in s essex.