So,
say the airline CSR (customer service rep)
and the flight attendants
and the cockpit crew (pilots)
refuse to make a "no eating XYZ food (e.g. peanuts)" on this flight due to LTFA passenger. . . .
and so you (the parent or allergic adult)
decide to
stand up at your seat and announce to the other passengers that there is LTFA person aboard and to not eat the food.
~ ~ ~
Potential outcomes?
Passengers are thankful and compliant. All is calm and good.
~or~
Some passengers are not so thankful. Even belligerent.
~or~
Flight Crew calls for security and you are removed from the flight by security, either before flight takes off or when it has landed. Perhaps you are refused boarding if you made your announcement in the terminal prior to boarding.
A passenger standing up to make an "announcement" on board aircraft could easily be perceived as interference with flight crew. Consider:
For the duration of the flight, the instructions of the crew are law. Literally -- it is illegal to disobey the instructions of a crew-member. This sounds like a bit of a stretch, but this interpretation has held up so far. The actual FAA regulation says:
14 CFR 125.328
No person may assault, threaten, intimidate, or interfere with a crewmember in the performance of the crewmember's duties aboard an aircraft being operated under this part.
And it turns out that disobeying any instruction from a crewmember constitutes interference with that crewmember's duties, whether or not you intended to interfere with his/her duties by disobeying.