 He or she must get trained. It's not cute when dogs bite, whether their teeth are small or large. They can hurt. Even if your little dog never bites somebody, there are a myriad of other extremely unattractive behaviors that are consequences of not training that little dog. You know, I run into this all the time. Well, it's a chihuahua, it's a schnauzer, it's a poodle warren. It doesn't need training. It's not a doberman, a shepherd, or a rottweiler.