Hi speedymetric .
As a new associated ref my self , i think its ok for appointed refs to make these mistakes & be in a nice manner asked by coaches in a nice manner after game , to clarify decision .( under age games are targetted by ref association appointees to let new refs learn )
Dont be thinking that these refs dont know how to / or havent played rugby before .
any player no matter what division profesional or not , if asked to ref a game ,,couldnt do it properly .
This is because reffing a game is totally different to playing .even if they played for 20 years .
Because there are so many laws , of how a game can or can not restart. & similar
Its a different kind of pressure ( some can handle it ,,some cant )
Just to be clear , when i say ask ref to clarify decision , im not suggesting coach view is that knock on never occured & ref says it did ,,sort of conversation .
But simply asking , how does a certain infringement restart , as your description above reads.
The parent lines man who took up challenge of running line , i say well done . ( assuming he stayed neutral ish )
His / her duties can only be to locate where ball went into touch & kick at goal duties only .
I too had a situation where , kick inside 22 kicked out on full from free kick that i had awarded ,where parent linesman in a nice ish manner suggested the throw in ( by his team ) should be where ball was kicked from . ( near 5 meter line ) & not out past 22 meter line where it went out .
I stuck to my guns & awarded line out to his team where ball actually left field
It did put doubt in my own mind , so when i got home i checked , & was found to be correct in my decision .
Hopefully your ref will do same & learn from his mistake ..
Finally
As a youth coach my self for last 4 years ,,i didnt realise how much of the laws i didnt know ,,( i thought i had seen & done it all ).
Reffereing has made me a better coach ( to be honest my passion is now referreing )
All ref societys always need more refs ,,i aplore all coaches & x players to give it a go