If you PM me,I can put you in touch with some breeders who have a litter or who have bitches in whelp.
Both parents would be fully health tested but of course you will need to verify this directly with the breeders.
Some breeders are happy to sell puppies to families with young children,it depends on the family situation.Some families have perfectly adorable and well behaved young children who will respect dogs, and you could have the local vicar with a crypt full of horrid little heathens.......Also bear in mind that breeders would occasionally have older puppies that they've been running on to see if they're show potential and very often they go as pets.
I would say not to concentrate solely on SM.
Ask about hearts too.It's all very well looking for a cavalier who won't have early onset SM,but look for dogs who have grandparents/great grandparents who have good hearts and some decent longevity behind them.
Also if you get a chance,try to visit some local Open shows where you can meet breeders who I'm sure would be happy to talk to you and advise.
The cavalier club www.thecavalierclub.co.uk have a list of shows for the year and perhaps contact the midland regional cavalier club.
If you have your heart set on a puppy where both parents are fully health tested,then you shouldn't feel you have to accept anything less.
Margaret C on this board has a list of breeders who say they health test their breeding stock.
Don't give up!
Sharing my sofa with Holly, Ivy,Lilly and Hazy.. and never forgetting our beautiful Daisy who reached the bridge too soon.