Hi, my 14 month old daughter has flu, is it safe for her to fly for 15 hrs to europe? I'm freaking out and worried about valsalva? What should I do?

Safe vs Appropriate. A 14mo can certainly fly when sick & the valsalva maneuver, should she do it, is no worse in a pressurized aircraft than on the ground. I would question why you would expose all the individuals on the plane & in the terminal to this infectious disease. If she has flu, she needs to stay home for a week & avoid spreading it to others who may become fatally ill.
, If she has the flu it is generally not advisable to travel, mainly as she will be exposing others on the flight to the infection. And she needs the same treatment and management that she is getting on land. Flight are well pressured so there is major respiratory issues with flying, but again if she is sick and has difficulty breathing, the slight drop in the pressure on board may cause difficulty.