If an 8 year old that was 4 foot 2 inches tall sat in a chair with the dimensions in the link below, what is the height between the ground and the top of their head?

We know that

The nine radii of a regular Nonagon divides into 9 congruent isosceles triangles
therefore
[the area of a regular nonagon]=9*[area of isosceles triangle]

[area of isosceles triangle]=b*h/2> 15*20.6/2> 154.5 cm²
so
[the area of a regular nonagon]=9*[154.5]> 1390.5 cm²

the answer is
1390.5 cm²