If you haven't already made plans, you may be better looking at Koh Samui as the weather will be much better then. August in Phuket is 'Green' season (rainy) with changeable, unpredictable weather and sea conditions.
If you do go to Phuket, please be sure to take note of flags on the beaches and don't venture into the water if they are red, even if it looks calm.
Some trips to the islands may be cancelled at short notice so be sure to have a flexible itinerary.
As for beaches, I agree with Richard - Kamala would be great for a family.