Carnival Cruise line - how are they?

We are taking a 4 day Carnival cruise to Ensenada and Catalina over Memorial Day weekend. We have two boys ages 5 and 7. What do we need to know? We have never been on a cruise before, so I"m looking for some insider tips! Is there a lot of motion? What will the weather be like? How do they handle tipping? Etc. Thanks!