Hi Leah and welcome to the board.
It's great that you are being considerate about exactly what you want in a dog and are taking the time to think all of this through really carefully. I guess the 1st thing I have to say is–most cases of SM, once the dog is symptomatic, are not easily manageable and I am sorry if that impression was given because I don't think any of us here with SM dogs would find this to be the truth, including Kate, who knows what an exhausting emotional experience it is to have a dog with the condition. There is also a study by the leading researcher in the area that, while informal, indicates that dogs with surgery do better than dogs with medications in terms of mortality. That said, the same neurologist and researcher also does not recommend surgery generally unless medications are no longer working. This approach differs from some other neurologists who believe that an early intervention gives the dog the best chance of as normal a life as possible. This is a very poorly understood condition in both humans and animals–one reason why human researchers are very interested in the research on Cavaliers–and regardless of how one cares for a dog with the condition, you do live with the constant worry about the condition worsening. A dog on medication will almost always steadily worsen over time to some degree, sometimes which can be managed on increasing levels of medications, and sometimes which can't. Some of the medications can contribute to eventual fatality as well, such as liver damage. Most of the pain killers gradually cease to be as effective and the dose must be increased. Many of our dogs are on 3 or 4 medications at a time.
I think you need to consider the lifetime picture. You could get a rescue Cavalier, but MRIing it would only give you a picture at that point of time, and this is a progressive disease that can happen at any time. Younger dogs are far more likely to scan clear. But in a study of over 500 Cavaliers, around 70% had a syrinx (had SM) by the time they are over 7 (and most of the dogs in this sample would have been breeders' dogs). The issue with hearts should be an equal concern, however, when you are looking for a breeder. You really want somebody who strictly follows the MVD protocol, whose parent dogs are heart clear, and you want all 4 grandparents to have been heart clear still when they had reached age 5.
You may also find it difficult finding breeders who will home a cavalier to families with children under about age 7. This is particularly true of rescue dogs, as you have found. Many breeders just feel that the dogs are quite small and that many parents, even the most well-intentioned, will allow more unsupervised interaction than is good for either the child or the dog. It's a matter of personal preference, and not true of all breeders, but something to keep in mind. A lot of breeders and people in rescue (me included in most cases, but not all!
) will suggest people hold off getting a dog till their youngest child is at least 5 or 6 because it is too often an unexpected and then unwanted extra burden on the parents and family management. But other families are great at responsibly managing the whole shebang.
If you want to Cavalier, I would hold out for a breeder who is doing proper MRIs on both parent dogs, who follows the MRI and the MVD breeding protocols and does other relevant health testing.
it is actually quite normal to need to wait a year or more to get a puppy from a really good breeder, and that's regardless of breed. My partner spent about a year finding a good breeder and then waiting For a puppy to get his German Shepherd, and is going through the same process again for a different breed–at this point it looks like it will be at least 7 months to wait for a puppy from the breeder he is interested in.
To be honest, if I wanted to have the best chance of a healthy dog over a lifetime, and you are really worried about the experience your children will have if a cavalier becomes ill, and this is a top priority, I would probably consider either a different breed (and as someone who ran cavalier rescue here for many years, I would also recommend a breed a bit larger than the Cavalier for children as young as 2 and 4) , or a mixed breed puppy from a responsible animal shelter (statistically, most likely to have few or no health problems compared to any pedigree dog). There is no way any breeder can give you less than a 1 in 4 chance that your Cavalier will eventually have SM to some degree, even if they do follow the SM breeding protocol (that said, the likelihood is also probably for milder versions, and perhaps the dog that is never symptomatic anyway, but there is such a high genetic likelihood now because the genes for the condition are very widespread across the breed).
If you feel strongly that you do want a cavalier, then I would contact Anne Eckersley or Laura Lang in the US (I believe this is where you are based). Both are breeders and are aware of other health testing breeders and can at least give you a steer to people you should talk to or they may have puppies themselves. But of course, it is important to talk through health testing with any breeder, including these 2. there is a very recent discussion about both of these people and why to contact them, so if you search for their names I know they will come up, and you can also get their contact details easily just by googling them.
If if discussing MRIs is a sensitive issue with ANY breeder, I would hang up the phone or walk away because that would tell me all I needed to know about their (poor) breeding program. It is not a sensitive issue with any health focus breeder! –the people that I know who MRI and work toward real breed health are proud of the effort they are putting into their breeding programs and work to MRI several generations to get a truly informative picture of their breeding program. They are all very eager to discuss what they are doing, what their approaches to breeding, and to give guidance and advice. Anyone who loves this breed will definitely want to work with such a breeder and not those who shy away from discussing something as basic and critical to the breed's future as health testing.