Have a look at the season you're going - your bargain might be because it's off season (ie wet).
That's ok, Thailand's still fun and still warm even if rainy.
If it was just adults I'd say go carry-on only if you're not trying to take sports gear.
There's really nothing you need for thailand that you can't buy there cheap. Wear your shoes, your 1 long pants and 1 jumper on the plane and you only really need undies and boardies and a few tees in the bag.
Kids complicate that obviously but if they're old enough then same goes.
On the other hand, some people like to buy a cheap suitcase there and fill it with clothes etc (@ $6 - 10 per item).
I would say thought that this hard to do if you're over 6 foot or the wife is anything over 'petite'.
If you're gonna book luggage then as dusta says do it up front or you'll regret it

Also if Airasia be careful around transfers, we had a major stress one time had a 3 hr transfer window (sounds like heaps right?) but the airasia terminal (in malasia maybe?) was like 5kms from main terminal
and you had to go thru customs to get there! grrrr... run run push push made it just..