In the USA, if you want your child to have their own seat, and they are an infant (under 2 yo) you must carry-on a car seat with certain specifications.

I am wondering if anyone knows if this is the case with Thai Air, BKK-LAX? We are planning on flying Premium Economy.

(I asked the travel agent but I've learned that you don't trust anything those people say.)

The same rules is for Thai airlines as for the US airlines - If you buy a seat for a under 2 year old, you will have to have a proper child restraint.

Last May, I flew from BKK to LAX (via Narita) on Thai Air and bought one extra seat for my not-quite 2 year old son. My wife held my 6 month old daughter. We were not required to have a CARES seat, or a child seat of any kind (and in fact, none of the staff on the phone or at the counter knew that a child seat made especially for airplanes even existed). They provided a wall-mounted bassinet for our baby for both legs of the flight, and were very accommodating about seating, food, free toys, hot water for formula, etc.

You are right not to believe what a travel agent says about in-flight regulations... Many of them have never even been on an airplane and are more than willing to pass on problems stemming from misinformation. My advice is to contact TG directly, and also to show up early for check-in so they can help you with good seating.

BTW, your child may be too big to use it, but the bassinet is a great thing to have because you can put all the kid's stuff in it for easy access (it's mounted on the wall in front of you at approximately chest level when seated)