It sounds like Kenzi is begining to get irritated. If there are any types of snaps and it isn’t in play, it could lead to a full out bite and more. Warning snaps, growling, and what not only go on for so long before the dog bites out of utter annoyance. In all fairness though, your older dog may be trying to establish the rules of puppy etiquette. While biting is not allowed, a bit of dog “talk” is intended to remind the pup of her position in life. For now, that means low man on the totem pole. Mothers establish rules, and your pup is no longer with her mother. Kenzi may be deciding on how far to let your pup be a pup. She is also an adult and may be demanding “respect” from the child, not to mention personal space, just like humans do. I do not mean to humanize the dogs, but it is a good way to explain. We don’t want kids bothering us 24 hours of the day, and neither do older dogs. Puppies before the age of 5 months tend to not be familiar with the subtle body gestures that adult dogs give when they have had enough. Adult dogs that were not properly socialized as a pup to all different ages of dogs throughout their lifetime will have a harder time dealing with a pup. So, it goes both ways.
Did you initially introduce them gradually and on neutral territory? If not, then you may need to rewind and start over. Initial introductions are of utmost importance in the dog world. You cannot just throw a couple of dogs together who have never seen one another and expect everything to be hunky dory for the rest of their lives. Just like how first impressions mean a lot in the human world; same thing applies to dogs. Maybe you and a friend could take them somewhere neutral and help to “re-socialize” them in a different location. Give them treats and gradually move them closer together and treat them the whole way. You want your older girl to associate the pup and her presence with good things. Take them on walks together (with another person walking one of your dogs) and do fun things that won’t always aggravate Kenzi. Everything needs to be positive. Take it slow!
While you are at home, keep them separated a bit more. If it were me, I would put the puppy in Kenzi’s least favorite room, or somewhere where Kenzi doesn’t spend a ton of time. You want to allow Kenzi to have the run of the house still, as you do not want her to resent the pup for being there. Treat Kenzi the same way you always have. She may be feeling “jealous” towards the pup and another dog being in HER house. Swapping scents sometimes helps too. Switch bedding from their crates to they get used to each other’s smell.
Without reading Kenzi’s body language and vocalizations, it is hard for us on this end to know what is going on and what could happen. I think that allowing Kenzi some personal space and time on her own without the pup getting in her face would be a good thing. Yes, they need to be socialized together, but if Kenzi is already making noises and movements towards the pup that could be read as aggressive, then there needs to be some time-outs away from each other.
The nature of the perceived aggression from Kenzi needs to be identified before you can “fix” an issue. Has Kenzi been to the vet for a senior checkup lately to rule out any pain issues or health issues? Many dogs who are in pain, especially older dogs, can get grouchy around other dogs, especially those with tons of energy like a pup. They are “not in the mood” or “feel up to it” to put up with a youngster. There was a lady who came into the clinic for an exam with an older Newfoundland and mentioned that her older dog was snapping and growling at a foster pup that the family was taking care of. They had never had that problem with other pups in the past. Lo and behold, we found an issue and later was diagnosed as cancer. More than likely, the older Newffie was reacting to his illness towards the pup. It is always a good idea to take current dogs to the vet for a checkup to make sure that they are “fit” to handle a pup or small, exuberent dog.
Has Kenzi showed any type of aggressive behavior PRIOR to the puppy? If Kenzi had behaviorial issues and very little obedience training prior to the pup, that could be an issue also. Keep in mind that if this is the case, Kenzi could turn around and teach the pup bad manners and then you will have a real challenge on your hands!
Just make sure that you constantly and consistently monitor the situation at all times and help assure that your pup does not infringe upon the dignity of the older dog. You may need to control the amount of time that they spend together. Wear the pup out a lot before allowing her to mingle with Kenzi. Both dogs should have crates, and that those crates are NOT used to punish; only to separate when need be. The need their own time alone, time with you, and time together. Get your pup into some positive training as you soon as you can, as you are undoubtedly seeing that she has some spunk and could be a bit strong willed. You have to teach her what is right from wrong before trouble occurs. They aren’t born with that knowledge! 🙂