Hi geostorm,
Depending on how full Camp Carnival is, they
may allow your 13 year old to stay with his sister in the younger age group. There will be a meeting the first day to sign up the children and you can ask then. Also, if there aren't many children on this cruise, they may merge the two groups anyway.
Personally, (and you
did say "What do you think?")

I'd encourage them to try their own age group for at least one day. They'll make lots of new friends!
Have a wonderful cruise!