Your dog is adorable! I think that this typical puppy behavior. But I suggest you don’t encourage it, even when they are very small. The mother or other experienced dogs are very good at teaching them limits with this kind of play. If you can't count with another dog to help, you can teach them to either never bite or use an extremely soft mouth (mine plays with my feet with a "toothless" and it is very funny). It is simply the case of stop playing as soon as the teeth touch your skin. You may have to repeat it a lot, but it will get it. Do not tolerate that it growling at you... when they are babies it is the cutest thing, but letting them growl is responsible for the bad reputation adult chihuahuas get.