Puppies and toddlers are a match made in hell. I speak from experience.
If I was you I'd rehome an adult dog. I have no experience of owning Cavalier King Charles Spaniels, but they were very popular on the site we camped at this year. I noticed the dogs more this year than I would have normally, because I was told by my dog sitter (DH) that my pup had been killed in an accident, just hours after we arrived . Being that dogs were on the forefront of mind the whole week, I paid a lot of attention to the three Cavalier King Charles that were there, and they were amazing.
I almost bought one myself. So calm, obdient and great with my dds. Dd2 was only 2 at the time, we couldn't keep her away from them.
Adul Staffies are also great with kids. One of the only breeds reccomended by the Kennel Club to be around young children.
But as already pointed out, dogs are hard work. Are you prepared for regular walks in the rain/snow/hail/wind/cold as well as the lovely summer walks? The hair all over your sofa? The training classes? The expense (insurance, food, fleaing, worming, neutering, training - it all adds up)?, accidents i.e. waking up to piss soaked floor?
And about the wee'ing and poo'ing everywhere, you have got to be prepared for the fact that when they are elderly, it might start all over again and there is not a damn thing you can do about it. My fathers Akita is getting on now and his bowels aren't what they used to be. My mum wakes up to a lovely, pile of shit that needs cleaning before she can make her coffee.
If you are willing to put up with all the above, go for it. To most people they are well worth the hassle.
I am having no end of troubles with my own rehomed tearaway (I didn't go to a proper rescue - my dog wasn't vetted or trained), but I wouldn't swap him for the world.