I still do not follow. What do you want from Thai? An apology or a compensation because your kids could have been affected (but they are still not affected I suppose)? Or do you just want to vent your unhappiness?

If you were to write to Thai now, they will ask what medical proof do you have that the child has chicken pox or why did you not report the matter earlier, if you had the necessary proof.

Do not get me wrong. I know how serious the matter is, if your assumption is correct. Perhaps the appropriate thing to do then is to inform the stewardess that you will not sit near them until a MD has examined the child because of what you suspect.

To take the matter up now with no possibilities substantiate your suspicion, in my humble opinion, is a little too late.