Kid's Undershirts (Underwear & Socks)

Worn against skin under everything else, an undershirt is a thin cotton knit in one of three shapes: a sleeveless tank, a short sleeve crew, or a long sleeve version for winter. Weight sits around 3.5 to 4.5 ounces per square yard, roughly two thirds of an everyday T-shirt.

Two jobs justify the extra layer. Sweat gets absorbed before it reaches a uniform shirt. That keeps underarm yellowing off a garment that costs several times more and gets washed less often. Warmth arrives from the air trapped between two layers, and one thin extra layer beats a single thicker garment for the same bulk. Rib knit, with alternating raised and sunken columns, stretches sideways and hugs the body, so it stays put under a tucked shirt. Plain jersey feels smoother and rides up faster. Fit here is meant to be close: a loose undershirt bunches at the waist within an hour and shows a lump through a school shirt.

T-Shirts in Kid's Tops are cut heavier and meant to be seen. A Bodysuit in Baby Clothing does this job for babies and solves the riding up by snapping under the diaper. That is why undershirts only make sense once a child is out of diapers. Sweatshirts go over the top rather than replacing this layer.

Cold classrooms and drafty buses are the everyday case. Pale uniform shirts stay opaque and stay cleaner with a layer beneath them. A child who cannot tolerate a wool sweater will often wear one over a long sleeve undershirt without complaint. Skip them in summer, where a second layer is simply hot. Never use one as pajamas: in the US, sleepwear above size 9 months and up to size 14 must either fit snugly or be flame resistant, and an undershirt has not been tested against that rule.

Check the length by having the child raise both arms, since the hem should still cover the waistband at full stretch. Look for a printed neck label rather than a sewn tag, as that seam sits directly under a shirt collar. Wash warm with the rest of the whites and dry on low. Replace an undershirt when the hem no longer holds a tuck, which happens long before the fabric wears through.