Humidity is trickier than dry heat because sweat does not evaporate as easily, so the goal is airflow, not less fabric. Muslin cotton is woven in an open, loose weave, and those small gaps let air pass through the fabric instead of trapping it against the skin. A single loose layer of muslin usually feels cooler than a lot of bare skin exposed to direct sun, since it moves with the body and dries fast if it gets damp from sweat or a splash of water.
We make our clothes from double-layer muslin cotton and wash everything before it ships, so it arrives soft instead of stiff like new fabric often is. The seams are sewn flat so nothing rubs against a toddler who is already warm and a little cranky, and the dyes are chosen to be safe for kids without any of that new-fabric smell. Muslin does wrinkle and crease easily, that is just how the weave behaves, and it is nothing to worry about.
Wash on a gentle cold cycle, tumble dry on low or just hang it up, and leave out the fabric softener since it coats the fibers and cuts down the breathability you bought the fabric for. High heat from a dryer works against you the same way, so keep it low. We size roughly newborn through 4 years, ship worldwide with tracked delivery, and take back anything unworn within 30 days if the fit is off.