I always use Hertz (because I drop my car off in Fort Myers and they have the most convenient location for me there... nothing against other car rental companies in Orlando).
The shuttle is fast and frequent during normal daytime hours... but yes, if you've got lots of luggage, it is a pain to get it onto the shuttle. Since you've got older kids, I wouldn't worry about it. They can help you carry.
The procedure:
Go to luggage carrousel and pick up your luggage from your flight. Look for "ground transportation" signs and go out to the street. There are taxi ranks, bus stops, and shuttle stops, none of them too far away from where you exit. Look for signs to show you where to wait or ask one of the airport employees... there are usually a few helping people to find their transport.
Go to the stop for courtesy shuttles and wait for your company's shuttle... They're clearly identified, much easier to identify than buses that do different routes. When yours arrives, hop on with your luggage.
How long it takes depends on how many more airport stops come after you. The Hertz place is quite convenient to the airport, but it can take a while if they have to make a lot of pickups after you.
One nice thing I find about the off-airport site, once you get there it's much more peaceful and less crowded than an airport... very calming before you get behind the wheel of a car. Make sure you get a map and directions when you're completing the paperwork for your car, since you need to know exactly where the rental place is.