They can't guarantee because they maintain a finite amount. At family heavy destinations like MCO, they are more likely to run out. If there is any way you can swing it, bring your own. Your child will be more comfortable in their own seat than the cheapest bottom of the barrel choices that the rental companies usually have. You also don't know the history of the rental seat. What specific model of seat and age is going to be determined by which company you choose. I'm also a big fan of using the car seat on the plane, it keeps the kids contained and they are generally more likely to fall asleep and sleep better in their own seat.