For safety's sake your 5yo should be in a booster. Most taxis will not have these. If you are staying on Disney property, you will be able to take the Disney transportation without any need for booster seats. If you'll need other transportation, you might also consider bringing your 5yo booster on the plane. If you have the full size seat booster you may look at purchasing the sit on booster seat. These are fairly cheap here at about $15.00. We brought our dd booster our last trip. It kept her where she could comfortably see where we were going. Although we could not use it on the plane, we had it with our carry-on.