Child seats cannot be in a seat that is offset more than 18* from the fuselage. This means that all child restraint systems are not allowed in the forward most cabin of 767-300s, 777-200s (edit: see below), or 777-300s. Whether or not someone would catch this rule (as it's a rather new one for FAs and GAs) is a separate matter, of course.

Edit: The manual's restriction applies on the 76A, 76N (767-300s); the 77E, 77J, 77K, 77Q, 77R (777-200s) and the 77W (777-300). I think the fleet updates thread has the information as to what the fleet codes mean.