TGR Jr has probably taken half a dozen round trips domestically (all under two) and same experience - never asked. International is irrelevant, obviously, due to the documentation requirements.

But I wouldn't attempt to bend the rules either. I'm also not sure that I would pay (vs CPU/RPU-ing) for a comfy seat in F if I was then going to have a 20+ lb child in my lap, 'legal' or not. If someone's going to be spilling my drink and shoving his/her fingers up my nose, I might as well be in E+.