Would you leave a 6 year old unsupervised with a hungry rat? No - what a stupid question. A chihuahua can cause the same degree of injury (life-altering).
Is there a significant difference in behaviour between a pitbull and a chihuahua? No - this study confirms what has been known for years.
Breed-specific legislation is utter bullshit. It clearly doesn't work, isn't workable anyway (breed is judged based on looks rather than genetics), never had any scientific basis, and has been proven to be detrimental to its own aims time & time again.