Far East can be dodgy weather wise at summertime but I have been there quite a few times and have had good weather in Ko Samui and Hua Hin in Thailand.
I think a 2 centre would be great , perhaps a week in Hua Hin , beaches there are not great but it's fab for shopping and there are some great hotels with lovely pool areas, and you can visit the floating market and bridge over the river kwai from there also go elephant trekking etc, I would follow this with 1 week in Ko Samui for the real paradise tropical island beaches, perhaps staying in Chong mon or Chaweng Noi as they are lovely areas with no seedy girly bars etc...
Hope this helps, I'm sure many others on this forum will have more good ideas , love shirleyV

As well as what Shirley suggests Bali has great weather! You could combine that with Singapore for even just a couple of days.
We have been 3 times to Koh Samui in the summer-great weather but we go for relaxation as it isn't really destination packed full of trip ideas. But have a look at Bangkok. One of our holidays we combined it with Singapore and there are direct flights to KS from Singapore. Daughter might like the zoo( both day and night) and the bird park. Chinatown and Little India are also great places. Have a trawl through the posts and see if anything takes your fancy!

Personally, I would go for Bali where the rainy season starts late November. I think we have enough rain here without travelling half way round the world for more. Another option would be the east coast of Malaysia but not sure if there would be enough for your 12 year old unless she's happy snorkelling, etc.

You really are spoilt for choice, there are so many great places in the far east.

I think for the first timer, Thailand or Singapore are the most accessable.

Ko Samui or Phuket probably have the nicest beaches. Singapore is great for shopping and siteseeing.

Wherever you decide to go, I'm sure you'll have a great time.