> Do you have a way of ensuring that the sling ropes work together?

I stole this from a photographer who used it for his backdrops. Take four pulleys: three singles and one double. Attach the double and one single to the ceiling or joist or whatever, far enough apart to support the boat properly. Reeve one line over one sheave in the double, down through a single, and back up to the ceiling. Screw hooks or eyes will work for both the pulleys and the upper tie-offs. Reeve a second line over the other sheave in the double, then across to the upper single, down through the third single, and back up to the ceiling. Slings are attached to the lower pulleys. Since both lines are coming back at you from the double pulley, you can handle them as if they were one, and cleat or tie them off them higher than your child can reach.