My guess is that it totally depends on both the airline and the flight attendants...my experience is this: I did NOT buy a ticket for my 1.5 year old and had her on my lap for landing (as she was irritable and it was keeping her calm and from crying) - I had a 'run in' with the flight attendant about the empty seat next to me and she started spouting the 'FAA law' about how 'EVERY ticketed passenger is REQUIRED to be seated during takeoff and landing'. So - I calmly explained that my dd was NOT a ticketed passenger but a lap child and she started explaining the dangers of having her on my lap for landing. But- the basics of what she told me was that because she wasn't ticketed - I had the choice to keep her on my lap but IF she had been ticketed - she would have forced me to put her in the empty seat.
Sounds like others have differing experiences though.
Sounds like others have differing experiences though.