Okay, mean how? They are not pushovers in a flock, but how mean can a 3 week old chick be, for heaven's sake? Barred Rocks are my favorite of all time. They are never at the bottom of the pecking order, true. They are not always at the top, either, but it depends on the others in the flock, too. The pullets are usually not aggressive to people, roosters can vary as they reach hormone stage, but yours are babies still.
Good around children? What can a 3 week old chick do to a child anyway? How old are these children? You do NOT want a rooster around toddlers, certainly, but most kids over toddler stage can deal with chickens, generally. Chickens are not dogs and male chickens should never be left alone unsupervised around any small child. For that matter, neither should most dogs. No breed as a whole is "good with children". They are individuals.
Another issue is are these hatchery stock, which can vary like crazy, or are these good quality true Barred Rocks, good breeder stock? You can have an aggressive strain in any breed, especially hatchery stock. I had some mean-as-snakes Delawares, which is NOT the norm for the breed, but they came from aggressive hatchery parentage. My heritage Dellie stock is fabulous.
I'd have to have more detail as to how you think they are being "jerks" to help much. I'm just sort of confused that a small chick is that scary mean and you're already judging an entire breed based on a chick 3 weeks out of the egg. Am I missing something?
Wait, did you put 3 week old chicks in with adult birds? You won't have any trouble with them for long because they'll probably be killed, if that's the case. I hope there are only chicks in there as well.