This is very embarassing. We were traveling with our 28 months daughter on a red eye flight from Singapore to Munich. She had her own seat and it wasn't the first time she's flown on her own seat (Jakarta-Bali). That time though, she was very sleepy and cranky (due to the flight taking off at 23:55) and refused to sit still during take off. We asked if we could have an infant seat belt but they informed us that it's against regulation because she's over two years old. We tried to calm her down with toys, stickers, even iPad but she still refused to sit and we were about to take off. Finally the head steward gave up and allowed us to use infant seat belt with her on my wife's lap. She's actually very small for her age and she's smaller than her 17 months cousin (who could seat on her mother's lap) so she could easily passed for under 2 years old.
During the flight, the head steward chastised us and said it's our responsibility to prepare her for the flight.
We had a few other flights with her on the same holiday with no problem.