We had a pair of Kelpie Xs for 14 years. Very protective of us and our children and very intelligent. Had them both from pups.
We now have a SA Ridgeback/boxer cross who is just gorgeous. She was abandoned and on the stray and lobbed on our door step one night at 9 pm about 5 years ago when she was about 18 months old.
If we had young children at the time I am sure she would have been perfect with them. Ridgebacks have a very good reputation with being around children.
Whatever breed you choose, you need to understand "dog talk". Dogs have the "pack mentality" and will usually accept their place in the pecking order. What is most important is to establish with the dog what the pecking order is.
Putting the dog through a dog obedience course is highly recommended. If your children are old enough (say 5 or 6) one or both of them should do it with the dog.
It is not for the sake of teaching the dog tricks or being totally subjected. It is about establishing a better relationship with the dog and the rules under which it will operate.
My 2 cents worth.

Edit:- I think popular opinion is that a Labrador is the safest dog to have around young children and that is probably right.