Hi and welcome to you and your new blenheim friend.
What you are seeing is actually very typical no matter the age of the adults! I think you need to give them more time. Many older dogs do not care too much for puppies and at the ages your seniors are at, they may not be much interested in playing anyway. Most likely over the coming weeks to months as the pup gets older, they will be far more friendly and playful together.
I'd highly recommend downloading Dr Ian Dunbar's free book on puppies, After You Get Your Puppy, at
www.dogstardaily.com/free-downloads as it is very helpful on managing a pup.
I would not get another puppy right now. For most people, having two together is not only difficult but makes for very difficult and slow training for everything from housetraining to obedience. Also trainers advise that each puppy must then be given equal and separate time every day -- not just 5 minutes but separate training, walk and activity times. Two puppies can become terribly dependent on each other which makes things difficult when they do need to be separated. Good breeders will generally not sell two puppies together or at least not without knowing a home can manage the separate but equal training.
Puppies really deserve having individual time put into them for the first year to 18 months of their lives. When the dog is around that age, is the best time to consider adding a puppy. Good breeders generally recommend at least 12-18 months for the youngest dog in the house, before adding a second.
Cost may be another issue to consider. Given the age of your older dogs and that almost every cavalier does get heart disease, another consideration would be that two older dogs that might need medications and some extra time and care and also a gentler environment. Most cavaliers will start to show heart murmurs by age 7+ so it is just one aspect to keep in consideration.
Finally -- if your older dogs are not excited about one puppy, having two would be likely to be at best, an unhappy doubling of what they don't like and a difficult adjustment, and at worst, very stressful for them at a time when they are the ones who have given you many years of happiness and devotion to your family :flwr:. I'd let them adjust to one, wait and see what the next year brings, and if you still feel it would be right to add another dog, then do so when the new blenheim puppy is 12-18 months old. A young cavalier puppy will far more appreciate time with you than another puppy and will be a better rounded and better trained adult.
Obedience classes, walks, visits to friends with dogs, dog parks, puppy socialising classes etc will all give lots and lots of opportunities for playtime that are far more valuable than having two resident dogs interact with a puppy, as well!
It is always great to find a breeder who is properly cardiologist testing and MRIing their breeding dogs for syringomyelia as well as testing for other important health issues (eyes, hips, EFS). When proper testing is done the chance of health problems drops or will be delayed and generally less severe.
