Well, it depends. How long are you staying? Where are you staying? Have you been to any of the parks before?
For some reason we do Sea World and Universal together on one trip and the next trip only WDW. Once in "the world" we stay onsite and use Disney transportation. HRH is onsite for US/IOA and they have free shuttle to SW. We never rent a car, too much hassle.
If you are staying offsite, probably just as easy to drive to SW as opposed to WDW. Your kids will LOVE Sea World. It's perfect for kids that age.