Lightweight, breathable organic cotton is the standout choice for keeping toddlers cool, since it allows air to move against the skin and absorbs sweat without holding onto heat the way some synthetic blends do. A looser knit or jersey cotton baby tee breathes better than a tightly woven or heavier fabric.
Pale or light colours help too, since they reflect rather than absorb heat, which matters for a toddler who is running around outside rather than staying in the shade. A light-coloured, breathable toddler tops option is generally more comfortable than a dark, heavier one on a genuinely hot day.
Avoiding synthetic blends matters more for active toddlers than for babies who are less mobile, since synthetic fabric tends to trap heat and moisture during play in a way that natural fibres do not, which can lead to overheating and general crankiness on a hot afternoon.
For the hottest months, a simple, breathable short sleeve or sleeveless top is usually the most comfortable option, with long sleeve pieces kept for cooler mornings, indoor air conditioning, or added sun protection rather than everyday summer wear.