According to many including the CNN weather site, Hua Hin has one of Thailand's lowest rainfalls, also it is very family friendly. Cha Am and Hua Hin are only 20 minutes apart, sometimes difficult to tell whether a hotel/beach is one or the other. Royal Garden Village and Golden Sands both have nice beaches, better than those near center of town. We\have been twice and also have been to Phuket and Samui, for families we prefer Hua Hin. 'Pleasant' nightlife that is very 'tame' compared to some other resort areas.

I've stayed at the Royal Garden Village in Hua-Hin as well and thought it was a very relaxing resort.

For Koh Samui make sure that you stay at a nice resort such as Central Village or their other one (forgot the name). Or some other nice property.

All of your planned places to go are great places--I'd keep them, definitely...

August should be okay as the weather is concerned...But mind you the rainy season doesn't really end until mid-October. But that doesn't mean that it rains all the time during the rainy season. And sometimes when it does rain it's short lived.....wouldn't worry about it.

From : A Bangkok resident.