M&S: bright plain-colour boys' t-shirts in very soft loose-fit cotton, with tiny subtle contrast tipping bits on neck & sleeves (cherry red on navy, orange on yellow, etc.) - ages baby to 12+ years - normally £3 each which is bargain enough, but currently 3-for-2.
H&M: little pull-on short leggings (for girls) - sort of knee breech length - in masses of colours.