@fu_man got to admit our jackets and wetsuits haven't been around in our front loader but at the end of the season the go around at the laundromat front loader
Back of the ski on the bottom of the lake while getting ready he can lift it off as the rope goes tight Just helps with the balance while he is getting ready to go
Tight rope or near to it he should be able to take off on a tight rope (if wants to a arm length hooked over a little finger so when it is pulled/fall from his finger the boat can take off) Look up above the driver somewhere. Rest the back of the ski on the bottom while he waits if the bottom is firm. Hands in tight on the hip. Don't step or stand on the ski it is more of a glide let the boat take you.
We started with the standard kids Connley doubles with the rear toe strap. This worked great for a starter.
We then looked for a 42 or 43 after seeking the same advice as yourself. At the speeds he will be skiing these sizes will work well as it will keep him on top of the water, then as he gets older and heavier the ski should start to work better. We are hoping that the ski will last for a long time.
The only issue is that they will probably want to go faster than their ability or at least that what we found.
Suitable to start with
20 years at a guess
Front Measure half way then put your foot in the binding and your ankle bone should be 1 cm forward of half way rear binding as close to the front one at an angle that you are comfortable with
Have fun
The larger kids ones with the rear strap worked for our six year old this year. When he was five he tried it on the boom and got up and nearly got up behind the boat as well but winter got in the way. They are really only going to go straight for the first few times anyway.
1 inch accuracy is available in agricultural auto steer this relies on GPS plus ground station. Speed may be an issue in the correction but the ability is there. That said I want speed control that can be retro fitted to all boats carby and EFI that just works for the every day skier.
In the end they will be fine. We have a bunch of kids that ski with us from 6-13 years of age and they ski on any of the shorter slalom skis fine. Though that been said we do not have any current top end small skis.
Here as some links on this site to junior skis enjoy the reading
http://www.ballofspray.com/forum#/discussion/6265/junior-slaloms-for-6-year-old-and-8-year-old-skiing-21-25-mph
http://www.ballofspray.com/forum#/discussion/8623/junior-use-wing
http://www.ballofspray.com/forum#/discussion/4306/advice-for-junior-ski-skier
http://www.ballofspray.com/forum#/discussion/5966/junior-specific-slalom-or-just-a-small-slalom
We went through this same thing. The advice was a 63 would be good as they are learning you will be going slow so a bit too large will be fine, then as they get older and you go faster the ski will still suit them.