Chickens are prey animals so they won't truly protect each other. You could get a rooster but most won't actually fight off an attack. They will call out a warning if they see a predator to give a chance for the flock to hide. If you have small kids and are just starting out with chickens, I'd recommend sticking to an all female flock first. Roosters can be human aggressive and can seriously injure small kids. It also can take the fun out of handling the chickens if you always have to watch your back for a rooster surprise attack. Not all are aggressive but young ones can have raging hormones.
If you have a guardian dog and will lock them up at night, you shouldn't have too many issues. Do your research on the egg laying capabilities, temperament, overall health of the different breeds. Personally, I have 6 different breeds in my flock because I couldn't decide. All the same age and raised together.
I bet the kids would like Easter Eggers. They're mixes and are generally healthier than high production breeds. They also are fun because you never know what they'll look like or what color eggs you'll get. Mine lay blue and green eggs.