Another vote for rescue rather than buying. I know that's not the question you asked but it is so, so worth considering :)
I don't know cavaliers much I'm afraid and my advice would be to 'shop around' quite a bit before deciding on one particular breed. You may miss out on the perfect dog by being too specific too early.
You probably already know all of this already but there are loads of good rescues that list their available dogs online so you can really take your time to find one that suits your family rather than taking a chance with 'what's on offer' at your local rescue (not knocking local rescues at all but there is no harm in browsing online to see what is out there. It can also help you decide on types and breeds to consider).
Many rescue organisations now place their dogs with fosterers for a few weeks before rehoming so they really get to know the dog, check housetraining, cat and kid suitability etc and make sure it's a good match for the new owner.
A good place to start is www.dogsblog.com - a UK wide resource for dog rescues with hundreds of dogs listed. You can search by breed, location, temperament etc.
www.manytearsrescue.org also has a very good reputation and lots of small to medium sized dogs available
Wherever you do decide to go, please, please do thoroughly check out reviews and credentials of any breeder or rescue. Puppy farming is a real and vile thing and people go to huge lengths to get you to believe you are buying a home-reared pup that is nothing of the sort. There are some dodgy rescues out there too (Ahem Dorset Dog Rescue
).
Wishing you all the best in finding your new companion!