You'll not go far wrong with a cavalier, I grew up them, and have the most handsome wee black and tan 6 month old boy myself.
My own experience is that they are amazing with children, even small toddlers. My boy lights up when he sees little kids, and is so gentle.
They'll be happy running mad all day at the beach, but equally happy but if it's been raining all week so you have to stay in and watch TV cuddling on the sofa every evening.
They are usually friendly to the extent that they will happily toddle off with any stranger who stops to pet them 
Now, they may not top doggie intelligence in terms of obedience and agility, but thats not to say they're not trainable.
They are a v sweet natured breed, and can be timid though this seldom leads to aggression, but they are sensitive, and don't like shoutyness!
My advice for the most important thing you can do for this breed is NEVER let them put on weight..even when doing puppy obedience training, they are so greedy, you can treat them with some food from their daily amount. They will do ANYTHING for food. Once they put on weight (which they do easily) the heart and joint problems can be more pronounced.
You do have to look after their ears (they get smelly, esp in warm weather) and their eyes can be teary. Daily eye/ear drops solves this though. Regular brushing and grooming also essential.
Also -if you're getting a puppy, make sure you can see at least the mother dog, there's been a terrible case here in N Irel where 24 of these gorgeous dogs were rescued in a terrible state from a puppy farm 