Carnival has special events for the younger ages. Your 18 year old may be outside the range for the O2 Club but your 11 year old is the right age for one of the Camp Carnival groups. I've heard good things about the activities they do. My daughter is a dancer with Carnival and they have special arrangements to meet with Camp Carnival to talk about ship life and professional dancing. I expect this is the equivelant of meeting with Mickey Mouse on the Disney ships.
There will be a Carnival Capers newletter each day that will tell you what is available. Be sure to ask a social host when you board about signing up for one of the children programs. Look for a crew member with Red Blazers, Blue pants, Name Tags,and a Huge Smile