It's  not that Caroline thinks that her son can't make a good king. It's that she doesn't trust her brother's schemes and his mischievousness when Rocky is involved. Also Rowan loves the little guy too much to let him get hurt. He may be the fun uncle who will let Rocky do whatever he wants but, someone trying to hurt that little boy would not end well on Rowan's watch.

Rowan ain't gonna hurt anyone as long as you take his first warning seriously and leave.