I purchased an XS (5/6) for my 5 year old and a S (7/8) for my 7 year old based off of sizing suggestions. They seemed to fit as far as the collar and arm lengths but were pretty long on the torso section.
My daughter who normally fits into a 4t and is very petite for her age, fits into the shirt okay at the top. She is about 33lbs and 38 tall for comparison for measuring. But as I said before it looks like a dress on her from the waist down. Hopefully it will shrink a little or at least get a little more use. Im sure we can take it in or at least tuck it in for the time being.
My son fits into about an Boys 8 now but still can wear some size 7 shirts. He is about 60lbs and 4 tall if that helps.
The material is great, as well as the colors being true to the picture. I will have to embroider a logo on each shirt and this material should not be a problem for that.
Overall Im happy with the purchase and got about 10 different colors per child. All the colors were great looking. I would purchase according to your childs current shirt size for an accurate fit. I think a smaller size wouldnt have hurt but we are happy with what we have.