On a hot night, less is more under a sleep sack. A short-sleeve bodysuit or a light one-piece is usually enough, even with nothing under the sack at all if the room is warm. Muslin is woven loose and open, so air moves through it instead of sitting trapped against the skin the way it does with thicker knits. That's the main reason parents reach for it in the heat: it's light and it breathes.
The easiest way to tell if your baby is too warm is to feel the back of their neck or their chest, not their hands or feet, which are often cool anyway. If they feel sweaty or their skin is flushed, take a layer off or switch to a lighter sack. A fan moving air around the room, light curtains, and a cotton sheet under them all help more than piling on fabric ever will. Dressing light and letting the room do some of the work is usually the better trade.
Our muslin pieces are double-layer, which keeps them opaque and a bit more substantial without losing the open weave that makes muslin breathable in the first place. They come pre-washed, so they're already soft on day one, sewn with flat seams so there's nothing scratchy against the skin, and finished with child-safe dyes. Wash cold on a gentle cycle, tumble dry low or hang to dry, and skip the fabric softener and high heat: both wear the weave down faster. We size from newborn up to 4 years, ship worldwide with tracked delivery, and take unworn returns within 30 days if something doesn't fit.