I have some pretty friendly chickens, but they still don't like to be held. My friendliest will sit on my lap, arm or shoulder sometimes while I'm out there, but act like they are going to be killed if I try to hold them. So for your kid's, try having them sit down with treats, and gradually they may get a lap sit from some birds. And a gentle chin/wattle rub, or on the chest, usually will go over better than being petted down the back. Anything that comes in over the top of them feels predatory to them. And you may have some that just don't want to be messed with at all. All of that is assuming that your kids are old enough to not be hurt or scared by a peck, and make sure to protect the eyes. Shiny eyes are a pecking attraction as is jewelry of any kind, and anything red like nail polish.