What sorts of mistakes is he worried about? (I didn't notice that comment earlier.)
Taking a towncar or rental car to the port means it's all under your control. You get to the port, drop your bags with the porters. Go drop off the car. (or drop off the car, take all the bags on the shuttle, then give the bags over to the porter when you get there) Go through to the security area and the checkin area, then board when it's your time.
Everyone sees their checked luggage when it gets to their staterooms later on.
It's the difference between possibly boarding around noon vs 2 or even a little beyond. For ME it makes a difference. Especially the two times we had concierge and there is a meeting in the lounge you need to go to before continuing on to explore the ship. For others it doesn't. But I am highly "value" driven, so the more hours I have on the ship, the more value I feel I got.
YES.
It just depends on what you and your child feel is a hassle.
With my son there would be a lot of talking ahead of time, but then there always is. Explaining what we're doing, what will happen, etc. If I had one checked bags and maybe a carryon each, with a rental car I would just go to the rental car return place (I'm specifically talking about Alamo or National b/c that's who I like to rent from), return it, then get in line for the shuttle with all our bags. If I were packing heavier, I would drive to the port, drop the checked bags with the porters (that's where the talking would be needed with my son, who would worry about the bags...then again, he would still worry when we handed them over to the porters after getting off the shuttle!) (and if we took the bus from the resort we would be saying goodbye to the bags even earlier), then drop off the car and deal with the car shuttle with just carryons.
For others, the rental car stuff would be a total hassle. It's all just personality, IMO. But we always have a rental car when in Orlando.
I've seen it at that price. I've also see it at over $70. Just depends.