Not safe and they probably wont let you use it. Even so, every time the seat belt sign goes on, per FAA regs, the baby will have to be removed. Better for her to keep the baby in the seat with her.

The smartest solution is to get two business seats and use a car seat. More comfortable for her. Safer for the baby. Both of them would sleep much better. 14 month olds love to move and don't do well being confined.