T-shirts for our small people :) are the same shape as Men t-shirts, just tiny. They come in sizes: 104cm (3 years), 110cm (4 years), 116cm (5 years) in our 3 colours: off white, nude and grey.
Measurements for each size are:
104 - width - 32 cm
length - 42 cm
110 - width - 33 cm
length - 43 cm
116 - width - 34 cm
length - 44 cm
Width is a the bust circumference: from under the arm part, over bust to under arm part.
Length is a measured from high part of neck to end of the t-shirt.

close up photo showing finishing of t-shirt.
Unisex.
slim fit, boy 6 years old - Kids size L/116

loose fit, girl 4 years old - Kids size M/110 (size S would be slim fit)
all depends what you prefer for your small Kid :)