Sometimes what sounds like a good idea in theory, turns out to be a bad idea in reality. I am sure your intentions were the best. Making sure the puppy is well suited to the new owners helps to assure the baby won't wind up being rehomed later; and a visit to the vet sounds like a healthy thing ;but, there are many diseases out there that without the vacine protection, the baby is really at risk. I also think that is why your Mommy is worried so much. By the time the puppies are 12 weeks, she is certain they are better able to fend for themselves, and less stressed at their leaving. I'm sure you will be fine when you get the puppy back; but i hope this experience shows you that it is just not a good idea. Let us know when you get the puppy back, and how things went. A better alternative might be to let prospective owners spend time with their puppies in a different room for awhile, but never allowed to take a puppy out for a "visit". Wishing you the best, and I know Tink will be a happy mommy later today.