Melbourne is a very small airport and easy to get in and out on. You take a few short roads and are then on Route 192 West toward Disney. There is a whole lot of nothing on that road until you reach St. Cloud - so make sure everyone goes to the bathroom, has something to eat or whatever they need to do prior to leaving Melbourne. It will be a good 45 minutes before there's a safe stop. Route 192 is not busy and is a 2 lane divided road (2 lanes going east and 2 west) in most parts. You will hit a ton of lights once you hit St. Cloud, but you'll soon see the mile markers. Route 192 turns into Irlo Bronson Highway and there are lots of restaurants and hotels along that road and you'll definitely start coming into more traffic.
It's a fairly easy commute but can take a while once you start hitting the traffic lights. I would plan on at least an hour of traveling. There are no tolls on the road. Make sure you have sunglasses since you are going east/west. If you have more specific questions, please don't hesitate to ask.