IT is not the breed it is the way it is handled and trained. I have had several Pit Bull mixes and NOT ONE was aggressive because I TRAINED them correctly. Currently I have a Pit Bull/ Staffordshire Terrier mix. It is a wonderful and loving animal and would be mortified if its teeth touched your skin. These dogs don't attack if trained correctly or unless they are provoked.Sorry, it is the breed -- or rather, the result of poor breeding. Any geneticist will tell you so. There was a selective breeding process that took place to make the dogs of today. These dogs have an extremely high prey drive and sometimes it is misdirected towards small children that they perceive as prey.
Some owners, like yourself, are much more responsible than others when it comes to breeding and training. Unfortunately, this breed got into the hands of the wrong people and unless you research the lineage very carefully, these dogs are more likely to be aggressive.
Yes Pure pit bulls are aggressive. If you don't know how to train and handle one you should not get one. Mixes generally are less aggressive. It depends on the mix. A mix with a boxer is asking for problems as Boxers are not the sharpest knives in the drawer.