My own view on this. Whilst your parents may well be entitled to some compensation, I think you have to ask yourself if the airline behaved reasonably. Delays happen. We have all had miserable experiences with airlines that have avoided their legal and moral responsibilities and in that case, go after them for every bit of compensation you can get. However, here TK rebooked your parents, provided a hotel and food. From what I can see, your parents were treated well. In that situation, I doubt I would pursue a claim.