Internal Flight South With Family

It's been about 5 years since I was travelling Thailand. Now returning with (Thai) wife and 18 month old daughter. Besides staying witht her family we'll have 4 of 5 days during which time we want to head south to a resort or island. I've been to Krabi, Ko Lanta and Ko Pang Ngan, but that was as a budget traveller.

First, any suggestions of a convenient suitable destination - was thinking of Krabi or Ko Lanta, ideally a beach bungalow or small resort - not into big hotels and still don't have a big budget. Need to consider we're travelling with an infant.

Secondly, though it would be nice to have flexibility I guess we need to book internal flights well before, especially as we're going over xmas. Any advice there?

Thanks for your help and sorry if it's so general.

Robin.