U12 game today and I had an absolute shocker of a match. Blue number 7 kept constantly questioning my decisions and I just didn't caution him in whole game althought he should probably been sent of twice. That reduced my concentration and my match control and I lost my focus and let a lot of borderline tackles go without even reacting or noticing. In the end neither one of the teams seemed to be happy with me. But at least I learned that I should let my mindset of: "They are so young I can't caution them" go and don't let the players affect my performance. Also a lot of "small" offences, like small kick to the shin guards or stamping (not with force or with intention) to players feet and I didn't know whether or not to call them or not because they were not affecting to play at all. Probably the worst match out of my around hundred matches officiated.
It's the mark of a good referee if you can be self critical without getting disheartened. I'm sure every referee in the world has been in your position one point or another.
As the kids are younger, the games can be more scrappy and you can be unsure of what's going on.
However it helps if you use your voice as well, even if you don't think it's an infringement. Typically if I see one of those challenges, where it is possibly a foul and possibly not, based on the atmosphere of the game and the tempo, I could be inclined to let it go, and simply say "careful there player, just watch the next one" in a friendly and non confrontational tone.
It sends a message to everyone that you've seen what has happened and are aware of it, and makes you look good because you're being proactive and keeping on top of things.
As for that age group, I tend to make a few allowances for tackles and such, it's rare you will see a yellow card offence, let alone a red, at that age. The players often mistime tackles but I find they very rarely actually meet the criteria for a reckless or excessive challenge.
If one player is keeping on with the niggly fouls, have a quiet word, then a public word (blow your whistle and call them over), third time blow the captain over and say that he's on his last chance and make it clear to everyone, point to the places where the other infringements were. On the fourth, give a caution. This applies more to men's football as a twelve year old captain isn't realistically in the best position to assist you (maybe consider having a word with the coach instead on the third one).
As for dissent, we'll that's not within the remits of the game. A mistimed tackle is one thing, but dissent is a conscious and deliberate attempt to undermine your authority. Give one warning, if you feel you can, then get the cards out. If a player is silly enough to get sent off you can say you warned him, so they have no ammo to come back at you with.
Take it on the chin, learn from it and come back a better referee next game.