There is a lot to consider with a dog that you want for specific work, especially if it's not exactly breed for it (as in, dogs are specifically breed to herd chickens). How much time do you have for this dog? What kind of dog to you want to live with?
Personally, I have an Aussie and can't imagine having anything else. Mine has enough herding instinct that he wants to chase things that move but he didn't grow up with things like cats and chickens so he gets a bit overzealous about them. I've known many that were great with everything--as long as they were exposed early.
Most herding dogs need a good bit of direction. They are smart. If you don't give them something to do they will get bored and find something to do--and you probably won't like what they choose.
In general...Border collies are the best working dogs. Always ready to go. But they are all about the work, not so much about their people. And they can annoy you for something to do (don't have an "off" button) or they pester animals (staring at the cat, waiting for it DO something...).
Heelers are typically aloof, 1-person dogs. Less tolerant of strangers than other breeds. I don't know much about their working habits, but I think they are pretty adaptable.
Aussies...they are good, but not exceptional workers (although there are overbred working lines that are more BC like and confirmation lines that don't have an ounce of instinct left in them). They have great personalities, definately one of the friendliest working breeds. I've also known people who won't get another aussie after having one b/c they are almost like children--pout if you leave them kind of stuff, people couldn't take the guilt! (my older one did this for 2 months when I brought puppy home
).
These are definately breeds that you want to research a breeder and make sure they have what you are looking for.
ETA: Might also want to think about how much your kids want to be involved with the dog. IME, heelers are the type that are fine with kids in their own family but not necessarily with others--the last thing I want it a dog that might nip or bite someone else's kid, or one that I have to lock up whenever anyone comes over (and the 2 that need this that I know off the top of my head are heelers).
BCs are fun for kids, they'll fetch alllll day and are totally into agility but it's about the play more than the person. Aussies are great family dogs--as long as they don't feel left out.