For booster seats, until a year ago (that was when we still traveled with one) yes, it was free for us. I dont think you have to worry as it states on their website:
Baby strollers/buggies/prams, wheelchairs, mobility devices, crutches and walking frames can be carried free of charge as checked baggage provided they are being used and/or an infant, child, elderly, disable or a wheelchair user that is travelling. Otherwise, a checked baggage allowance must be prebooked. These items, except for crutches and walking frame, may be used until the boarding gate before they are checked into the hold of the aircraft. Upon arrival these items will be available on the baggage carousel at the arrivals hall. We will not be able to retrieve these items at the aircraft site and bring it to the aircraft door or passenger steps.
http://www.airasia.com/ask/template.do?id=174
Yes, it does not state booster/car seat, but I dont see it as an issue.