This may be common knowledge of course but I've only just discovered it and I know people are always looking for them.
They have boys' shirts with short or long sleeves, girls' just with long sleeves I think, and polo shirts - all only in white.
Also boys' trousers but only in black.
They haven't got all sizes on the website, but they are doing 3 for 2 at the moment (I love it that the caption is "Back to School" in June )