How long is your day in port? Most port stops seem to be approximately 7-4. If that's the case, you could probably safely plan on being off the ship by 8, maybe a little bit earlier. Magic Kingdom is an hour each way if everything goes perfectly, and Orlando traffic rarely goes perfectly, so let's say you get there at 9:30 or 10. That would leave you with about 3.5 to maybe 4.5 hours in the park before you'd need to be back on the road to be on the ship by all aboard at 3:30. Aside from part transport, remember that you need park tickets (at probably $120 minimum per adult, the baby's free) and a park reservation. This is a very long day for a fairly minimal amount of park time. Please think about whether or not it's worth it to you.