I took 4 boys (6,6,10,10) to the O2 Beach. According to the kids 'its was brill'. They had a fab day - def the best of the hols!! And it was free.
It's a massive circle of sand with fake palms, with a decking path around it surrounded by carpet. All the boys were given a beach ball, borrowed buckets and spades and the grown-ups borrowed deckchairs - all free. There were pirates floating about playing with the children and organising music and craft activies too. They also played volleyball.
We got there at 11.30, took a picnic and dragged them away after 4+ hours.
Open 10am-10pm. It ends 27/8/7. Jubilee line - North Greenwich. Perfect for a rainy day.