Sonny will disagree with me here, but since he refuses to try a CCL ship to compare the food, I would have to say that they are at least equal in that area. In fact, I might give Carnival a slight edge in food quality, but only for the choice in the desserts. I also lean towards Carnival a bit more only because CCL was my first cruise experience so I tend to compare everything to how CCL does it. But the RCCL ships are great too.
You can find my Mariner review here
but the quick summary is that they are in my opinion fairly equal product offerings. With that being said, it comes down to cost. RCCL tends to be a little more expensive then CCL. If you are looking for the rock wall and ice rink, then the extra money may be worth it for you. With an 8 year old, maybe those things are enough to warrant the extra cost. When I'm paying for a faimly of four, the extra cost gets multipled by 4 so it becomes a deciding factor for me and the wall and the ice rink are not enough of an edge to overcome the extra cost. If the money is not a huge issue, then the Mariner's extra activities give it the advantage.
My girls (13,11 at the time) enjoyed the Mariner as well as the Spirit, both cruises offer lots of activities for the kids. I like the Spirit class ships because they have a great layout, so you will enjoy the Legend.
So I guess my answer is you can't go wrong with either, so choose the one that has a price point and an itinerary that suits you.
If you have more questions after you read my review, feel free to PM me and I will answer them.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013