I ref a lot of youth football and i am quite strict on fouls,Barges are always a moaning point. A player can send another player flying to the ground and then claim it was only a shoulder ref, normally followed from a parent "theres nothing wrong with Toby keep on doing it". I would have no hesitation to get my cards out at 9v9 upwards. For younger age groups only for violence like what was just mentioned or very bad language I would use cautions. Bad fouls I would manage at 7v7 and ask for the player to be removed.