OOoh you lucky thing. I LOVE thailand. Have spent many months there in my young, free and single days and we went there March this year as a family (with 20 month old). I have travelled over many parts of Thailand and its all different and beautiful in its own right. However, with kids I recommend Koh Samui. Check this amazing place out:
www.rockyresort.com
it it british owned and managed with lovely thai staff. they seriously cannot do enough to help you. fab position, best to hire a car really so you can come and go as you please ( a really nice air con jeep only costs around £9 per day). the food at this place is great, you get massage on beachside for a few pounds, there are lots of wonderful spas nearby (def go to Tamarind spa)
we did it independently. booked accom through rocky resort website. booked flights through bridge the world(i just did loads of searching and they had best price).
good thing (but maybe sad to say it) there is a big tesco on the island so plenty of nappies, kids toys, sandals, biscuits etc. etc,
the only thing i found difficult with a child is thai people seem fascinated by western kids (esp little ones) and they want to touch them and kiss them. i had been warned about this but was a shock when thai woman at hotel kissed him on lips YUK - she kept trying to do it and pick him up so when i saw her coming i would quickly pick him up and turn my back!! she got the message. she meant no harm but as i was told they would never do it to thai kids so have no right to do it to others.
def make sure you get air con rooms, and if you want any more info i can talk on thailand forever
you will have fab time! 
pinkmamma