There are many conflicts in the temperaments and other characteristics of a Chilier. Mixed breeds are always a gamble even when they are similar breeds, and this particular hybrid will present a wide range of aspects. You'll never know just what you have until your Chihuahua Cavalier King Charles Spaniel mix is mature.
While these dogs are smart enough, they may show some not-so-smart talents due to their stubbornness or objects of other focus. The parent breeds have their own desires and strongly prefer to do as they wish. You should expect the same in Chiliers, but in different ways. Some may simply ignore your commands while others may not even be aware of them.
Likewise, you can expect anything from cuddly loyalty to brazen defensiveness in these dogs. One way or another, however, they will push hard to do what they want. Still, you'll want to avoid harsh types of "training" as they are very sensitive dogs and are much better trained with gentle firmness.
Chihuahuas are strong dogs, especially for their size. The Spaniel, while sturdy, is a bit low on the strength meter even as they do have endurance. You might get a dog that, like a Chi, is better suited to some strength-related activities. On the other hand, she may be the kind of dog that prefers tracking exercises or long walks. In any case, you'll want plenty of agility activities.