My daughter has to get a root canal after that has to get on an airplane will she have A LOT of pain due the pressure what if the root canal fails?

No if it is finished. During a root canal dentist removes the dead nerve that occupied the nerve chamber inside the root and fills the chamber with l filler material. This might take one or several visits. Between the visits the hollowed chamber is medicated. At this stage the change in the air pressure can expand the air in the chamber causing pain. When the root canal is finished the chamber is densely filled,