Pediatrician has referred my young cousin to a pediatric cardiologist, should we be worried?

Not necessarily . Most referrals to a pediatric cardilogist are for a murmur, and many of those are benign. Also depends on how old your cousin is, and how his or her general health is. If ur cousin is healthy then don't worry, but they still have to be seen by the peds cardiologist as recommended by the pediatrician.
Depends. Many times the pediatrician has identified a simple benign murmur, but realizes s/he will be second guessed by all the in laws if the nature of the finding is not confirmed by the "specialist".Other times there is a question of whether more evaluation should be done.I was quite happy with what my stethoscope told me but I also understood the reality of dealing with extended families.
Depends. There are many reasons for referral to a pediatric cardiologist, so it would be necessary to know why he was referred in order to answer your question. Most commonly it is for evaluation of a heart murmur, which most of the time is "functional", a benign murmur that is common in children.