They are pretty good with kids. The thing you'll need to watch for is their herding instinct. Blue heelers might nip at kids' ankles to try to herd them somewhere. This breed was developed to help herd cattle, so if you have cattle needing to be herded, these dogs can definitely help with that!
We had a blue heeler when I was growing up, and he was never aggressive with us. He was very energetic, loved to help out on the farm. If you get a puppy, you can easily train him how to behave around children.