Carnival does not require kids to be potty trained in order to participate in their kids programs. The do require however that parents provide all changing supplies (diapers, lotion, etc). They are the only cruise line that allows non potty trained kids to do this. The others do not make exceptions (even if the kids just wear diapers in case of an accident). I've had good reports from many of our readers about their experience with kids and Carnival. The caveat is that some of their cabins can be small for more than 2 people, especially if one is a wc-users and their drainage in the roll-in showers is horrible. But the kids have fun in their kids program and as I said all kids can participate.
Editor, Emerging Horizons
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***edited to remove commercial reference***