I'm another who is not convinced it's a great idea going for a puppy with a child who is nervous of some dogs. Puppies jump up and bite, they steal and destroy things and are pretty much the definition of dogs that aren't well trained! As a puppy one of mine used to literally launch himself at my face, snapping like a mad thing (he connected on a couple of occasions as well...), when he was giddy. He's the loveliest, gentlest thing these days but, my god, he was a complete shit when he was little. He wasn't remotely unusual either, plenty of puppies are equally objectionable.
Sometimes getting a puppy (or adult dog) when a resident child is nervous or scared of dogs works out well but also sometimes it doesn't. It's not that unusual to see puppies or dogs being moved on after fairly brief periods because the scared child isn't coping.
Bear in mind your DS will also need to be able to visit prospective dogs/puppies (reputable rescues and breeders will both insist on the whole family meeting any potential dog/puppy) which may mean being in very doggy environments. For example it's common for breeders to have multiple adult dogs who they want to see prospective puppy owners interacting with before allowing them to see the puppies.
I'd also recommend really doing your research into the breed/crosses you've mentioned.
The CKCS is highly prone to some very devastating health issues, mainly Mitral Valve Disease (MVD) and Chiari Malformation/Syringomyelia (CM/SM). Both are worryingly prevalent with UFAW estimating that almost 100% of CKCSs will have developed a heart murmur by the age of ten and that the proportion of the breed over the age of six with CM/SM visible on an MRI could be as high as 70%.
Screening schemes and breeding protocols do exist for both conditions (MVD and CM/SM) however the nature of the conditions mean that these can only reduce the risk somewhat, not completely eliminate it. Even if a breeder is doing all screening and following the breeding protocols to the letter there is still a risk of any resulting puppies being affected. The prevalence of these conditions within the breed can also make avoiding them whilst keeping inbreeding coefficients at an acceptable level very difficult and that brings its own set of problems.
Although the breed is in desperate need of some outcrossing it's important to be aware that MVD is also potentially a concern in miniature/toy poodles and CM/SM has been seen in mini/toy poodles, bichons and CKCS crosses with these breeds. This means that it's vital that breeders of such crosses are still screening at least the CKCS parent for both and is still following both breeding protocols.
It's rare to find a breeder of pedigree CKCSs who is really doing absolutely everything the should be to maximise the chances of good health in their litters and it's even rarer to find a breeder of crosses doing so and this is really not a breed you want to take any gambles with.
If you are still interested in the breed I'd highly recommend the following links:
Cavalier Health has extremely extensive amounts of information about both MVD and CM/SM as well as the myriad other conditions present in the breed.
Cavalier Matters is also good and a bit less overwhelming.
The UFAW CKCS pages covers some of the most common heritable conditions in the breed.
Clare Rusbridge (Professor of Veterinary Neurology at the University of Surrey and Chief of Neurology at Fitzpatrick Referrals) has a chunk of her website dedicated specifically to CM/SM with loads of great information as well as links to lots of other useful sites.