Interesting. I have a slight variation.
The first time I rigged my Jackline, as you did, I had two, and had the commercial 6' tethers. It was clear that if a kid went over, it would be bad even tethered, they would be dangling possibly in the water.
So I made my Jacklines go down the center line. It's actually two running in parallel either side of the mast. They stop short of the bow.
I then made my own tethers with just a 3' piece of webbing with two loops. For under 100 pound kids, I think home made ones are as strong.
The net result, because I was able to customize my tethers,is that the kids actually have to pull slightly against the Jackline and tether to sit on the rail like your girls are in the picture. Seeing those lines under tension in that spot makes be feel much better than the 7-8' of combined Jackline and tether slack.
Three boys, 200 sq ft of fiberglass, tropical beaches