Young teams can actually be quite difficult to handle sometimes, especially if they're a team of smart alecks. I once refereed an U/12 team that I really struggled with, because I had never faced that level of dissent and unsporting behaviour from that age and I was caught off guard. The thing about this team, fairly high level local team (by the way, we play full-sized games from U/10 up), local A grade. But I could see they were well coached into thinking they were a much higher, older grade in terms of how they act. I mean, the way they commit cynical fouls, even how they try to get under the opponent's skin. I even found there was a lot of dissent, but it wasn't just mindless whinging - it was like they had been coached into how to dissent effectively to get under my skin.
This team copped a couple of cards for dissent, and I think a RC for VC. I refereed them a few times later in the season, and they'd normally cop some for dissent (one particular scenario, the player clapped at the first card - I had a strong word, but this was a team that knew how to abuse the fact that a lot of referees would think 'oh, they're just kids' and keep the cards in the pocket). After the match, the opposing coach said he was really surprised I didn't respond to the clapping with the second card. Regardless, I didn't lose control of these later matches.
Basically, the kids were a bunch of prima donnas, and the parents were exactly what you'd expect from this. I learned from that game is that sometimes the best thing you can do is dish out the cards.
Personally, my tolerance level for dissent, aggression and clearly intentional unsporting acts is much lower at these ages than at higher grades. I'll still try to use roadblocks, but I have no problems with booking kids who want to mouth off - and this is the age where they love to test authority.
Try to get a feel if they're just complaining because they're upset, or if they're trying to impress their mates. At times, calling a player over (to neutral territory) can help calm him down as he gets a little embarrassed while everybody watches him walk.
If it's personal, public or persistent it needs to be dealt with. If they're just appealing, keep an eye on it and if it's happening a lot, pull the players up. It may be that simply talking to the players can be enough to handle the appeals, or maybe not.