Unfortunate for the child, however, the parents are being too precious.
If this was so important to them they would be researching if SIA was a nut free airline, and apparently it is NOT.
https://www.singaporeair.com/en_UK/s...h-nut-allergy/
Therefore it could be argued the parents have been negligent, failing their due diligence?
Exactly the same as buying a packet of biscuits and marked "May contain nuts", the parents buy it then complain later it caused an allergic reaction. Parents have failed their parental responsibilities.
Time to find another transport option for the family? Or maybe a report to child protection is in order?