Help me out here...
Reach = distance from the saddle to the handlebars
Crank Shorteners bolt onto 170mm adult-size cranks to reduce the size of the pedal rotation to something that is more appropriate for a child, e.g., 135mm, 145mm, 155mm
Kiddie Cranks are an extra set of kid-size cranks that bolt to the seat tube above the tandem's fixed cranks for very small children and, to the best of my knowledge, you can't always use these on softride equipped tandems.
If you're trying to get the handlebars closer to your child's saddle position you would just need to procure or fabricate a super-long stoker boom.
If you're trying to shorten the seat height to allow him to reach the cranks, again, softride bikes have some design constraints with regard to how much you can raise or lower the seat height.