We used a car seat on the plane, whether we needed it on the ground or not, until ds was four yrs old.
This site explains airline travel and child safety. Beyond the safety factor, DS would have easily unbuckled himself from the airplane seat belt so using the car seat saves us from a lot of hassle keeping him belted in during the flight when the seat belt light was on (and we've been on x-country flights where it's been on for the majority of the flight due to turbulence).
For a van ride, or any vehicle with seat belts, ds definitely
would be in a properly installed car seat. As a pp stated, the laws of physics don't change just because you're on vacation. I've seen plenty of car accidents on the roads around DW, and we were even involved in a very minor collision years ago while driving a rental car, ourselves. Also,
Florida child safety laws actually do
not exempt children from being in a car seat in a taxi, from what I read. The law itself is still pretty lax, IMO. If you do take car seats then you'll have the dilemma of what to do with them while at Unversal. If you rely on Mears or other transport to provide them, then you don't know what they will show up with - whether it will be the appropriate size seat or even in good condition. Not something I personally would want to rely on.
Other options would be renting a car for a day - we've done this.
This site has more info about where to rent cars from at WDW. The other option is just to skip Universal. Personally, I didn't think it was a
great park for little kids anyway. There is certainly plenty to keep you entertained at WDW. You might want to save Universal for when your kids are a bit older.
hth
