I think you'll find that the line one person prefers is going to differ from the favorite of another. Having cruised both CCL and RCCL, I prefer RCCL for a lot of reasons, but I don't have young children to consider.
I think that since you're cruising with children, you need to look at what the ships sailing from Galveston have in terms of entertainment for the kids. Both lines have kids/teens programs. Both have the requisite pools. . As I recall, only Carnival has the 2-story water slides (and I'm not sure that all ships have those). Both have sports courts (basketball). RCCL has rock climibing walls on a lot of their ships (I don't know the age limit.). Some have ice skating rinks, too. . .
Food is probably about the same on both lines, although I would give Carnival higher marks for their buffet offerings. And, the fact that they have a 24-hour pizzaria is great.
Hope this helps!