Are the good guard dogs?

Absolutely NOT!!

The ​ Cavalier King Charles Spaniel is perhaps
the most inviting of all toy breeds with strangers
and will warmly welcome anyone who it sees. 

This dog seems to think that everyone is a potential friend.  Many Cavalier King Charles Spaniels will provide a bark to let their owners know that
someone is at the door, but this bark is more
of a, “There’s someone here to play with me,
please open the door so I can say hello,
” than it is a warning. 

Few breeds are less suitable for guard dog
duty than the Cavalier King Charles Spaniel as
this breed is more likely to lick an intruder to
​death that it is to cause them any real harm.