alt Hacker NewsI've only done one return flight with Thai Airways (LHR-BKK) with a child seat, but it's only allowed by a window seat or in the middle seat in a 3x3x3 configuration. Probably standard but I didn't check for a while.
(Sorry, meant to reply to parent)
Pretty much the same experience I've had on other carriers. Where you can/can't put them is pretty logical.