I just returned from my first trip to Thailand and even now don't feel qualified to answer many of your questions, but wanted to recommend the Dusit D2 in Chiang Mai because I enjoyed my stay there so much. We paid a bit extra for the club level rooms and enjoyed the "free" cocktails and snacks each evening in the lounge. It also has a pool which might be nice for the family during the summer and is very close to the Night Market and many restaurants.

We also did a visit to Patara, an elephant farm. It seemed like an excellent operation and is working to rescue/preserve elephants rather than exploit them for the enjoyment of tourists.

there is no special problem with july travel.. june and july are similar.. there will be some afternoon rain but it usually ends in 1 hour..

riverside hotels in bkk: look at 2 bedroom suite at chatrium riverside.. maybe the ibis hotel??

we loved peace resort in samui a few years ago.

consider hua hin for beach time--no costly flight required.

IMO, Koh Tao is a long way to go with children, and it's also one of the least scenic islands (although nearby Nang Yuan is pretty). To get scenery like Phi Phi you will want to take a day trip to Angthong National Park, which leaves from Samui.

I did my scuba diver training on Samui in July. At that time, it didn't rain every day, and if it did it was only the typical afternoon thundershowers.

For accommodation in Chiang Mai I highly recommend http://www.3sisbedandbreakfast.com/ it is in a good location on the edge of the old town, great for seeing the city. It is great value with very helpful staff and some good restaurants close by. We stayed in the B&B rooms which were very spacious, good air con etc. they also have some lodges which may suit your family better.

Totally agree with MichaelBKK re Koh Tao, it is along way to go for a few days and unless you are a diver, there is no reason to really go there. Flying to KS used to be expensive but not sure if this is still the case. With the time you have available, rhkkmk suggestion of Hua Hin warrants further consideration. It is now VERY westernised but has some nice hotels and some great day trips round and about. Stayed in the Sofitel Central many years ago which is a very atmospheric, colonial style place that was used as allocation for filming The Kiling Fields.

The humidity is an issue but most people soon get used to it. I find avoiding air on during the day helps acclimatisation process.