The metacentric height changes linearly with the scaling factor. For the same righting behavior the paddler’s center of gravity should also move in the same proportion. Children are somewhat chunkier than adults which should improve stability.

I would guess a Great Auk is about right for a 180 lb adult so scaling it by 75% would suit a 75 lb paddler. The stability of a given design increases as load decreases, so using a larger scaling factor will increase stability as well as accommodating growth.

A 60% factor would suit a 39 lb paddler. An average child of 3 years weighs 33 lb - he might outgrow it before you finish! An average child of 11 weighs 75 lb so the 3/4 scale should be OK for a few years even if he is a big ‘un.

I’m not sure what the strip thickness and depth should be but these would probably need to be scaled down to keep the weight in proportion and simplify bending.

Some thoughts:
A shorter hull is easier for an indoor build.
A canoe might be safer for a young child than a kayak- less chance of being trapped.
A 3 mm marine plywood hull would be create less dust than a stripper.