good beach near SW London?

[basildonbond]

I'm planning an end-of-the-summer-holidays day trip to a beach with my dc and their cousins (7 kids in total ranging from 7-13)

Can anyone recommend a good (sandy) family-friendly beach within an easy drive of south-west London please?

thanks :)

I live in SE London and we go to Broadstairs, it's a lovely beach with lots of sand, if you want something quiet then go to westbay which is not far from margate and Broadstairs, also has a lovely beach and rock pools for little ones to play in.
I go every year and the kids love it.