Can you take a cab with kids & no car seats in Disney???
I found this on the Florida DMV site:
Children 3 and younger must be secured in a federally approved child-restraint seat.Children 4 through 5 must be secured by either a federally approved child restraint seat or safety belt.
So, it appears as though your 5 year old would be fine with just a seat belt, but your 18 month old will need a car seat. Since a cab is a car, and not a bus, I'm pretty sure these rules would apply. I don't think this applies to the buses because there are no seat belts on the buses to begin with.
We just came back from Disney at the beginning of Feb and used Happy Limo