Players, coaches & parents will react like you have violated their favourite aunty if you dare to show a card to a player below u18's.......
Ignore them, carry out the procedure correctly and bask in a self satisfied glow of knowing that you are the only person in a 100 yd radius (maybe less if with neutrals but not always...) that has a firm knowledge of the LOTG, having applied them fairly & without favour in the diligent execution of your duty.
The whiny moaning ingrates (players, parents and coaches) are simply not accustomed to a referee who knows their trade well and isn't afraid to show it. Don't worry, they'll remember you for future games, and eventually, the moaning subsides into a grumpy grudging acceptance that they can't intimidate you in doing what they think is the right thing.
If you want to blame anyone for this (and you really should try to avoid a blame culture) blame your colleagues who are more interested in being "popular" (and thus getting all the cup final appointments) than in doing the job properly (and thus getting bugger all cup final appointments) who pander to the nonsensical beliefs of players, parents and coaches.