IME flights are sometimes busy during holiday times (Thai and western holidays) so it may be worth booking before you go, particularly if there's more than 1 of you travelling.
You could get the train, but the ferry to samui is a bit crowded and the sea can be rough and the backpacking crowd can be a bit lairy - oh to be a backpacker again!!!!
If you want somewhere a bit different, try Krabi or Koh Lanta www.lantainfo.com/ which is absolutely fabby and laid back and very child friendly! If the budget is accommodating, the Pimilai resort is just amazing - we finished a tour of Thailand with a 5 night stay there a few years ago and my DC (now 17 and 15) still harp on about how great it was!
I'm told by Thai and continent hopping friends that Phra Thong Island is a good place to head - but have not checked it out for myself.
I'm vvvvv jealous - I'd love to be heading to Thailand this christmas.