I would give up the idea of a Pitbull unless you want to move to Northern Territory or ACT and it's illegal to import them - they're on the federal customs banned dog list. And you'd never be able to take your dog to Victoria.
Staffies (SBT) - especially as puppies are a high energy dog and you will need to take them places to meet lots of different dogs and children as often as possible eg daily to encourage friendly behaviour.
I'd give up on Blue too. Skin problems with those.
If you haven't got time to train your dog and exercise it, asking it to play nice with your new family is a bit much. You'd be better off with a british bulldog - all those I've ever met are completely friendly and they don't need much exercise.