Muslin is woven loosely, with small gaps between the threads. Those gaps are what make it breathable: air moves through the fabric instead of getting trapped against the skin. That's why a muslin long sleeve doesn't feel like a regular cotton long sleeve. It's lightweight and airy, so it can help keep a baby's arms shaded and cooler in direct sun than bare skin would be, while still letting air through.
There's no single right answer, it depends on the situation. Long sleeves are worth having for time in direct sun, windy beach days, buggy rides, or rooms with strong air conditioning where bare arms get cold fast. For everyday play at home or in the shade, short sleeves or a sleeveless piece work just as well. Our pieces are double layer muslin cotton, pre-washed so they're soft from the first wear, sewn with flat seams so nothing scratchy rubs against little arms, and finished with child-safe dyes.
Muslin creases easily and softens more with every wash, both are normal, not a flaw. Wash cold on a gentle cycle, skip the fabric softener, and tumble dry low or hang to dry, high heat isn't kind to the weave. We size from newborn up to around 4 years, so you can size up for a looser, cooler fit if you're layering for heat. We ship worldwide with tracked delivery, and if something doesn't work out you can return it unworn within 30 days.