A good rule of thumb is two to three outfits per day, then one or two spares for the whole trip. Newborns go through more changes than a two year old, since feeding and diaper leaks happen more often in the first few months. If you're headed somewhere hot or humid, count on the higher end of that range. Toddlers who are potty training or eating on their own also tend to need an extra outfit some days.
Muslin cotton helps you pack lighter, since it's woven with an open, loose weave that dries fast. If you rinse an outfit in the sink at night, it's usually dry by morning, so you don't need one for every single day of the trip. The fabric is soft from the first wear because it's pre-washed before it ships, and it gets softer with every wash after that. It creases in the suitcase, that's just how muslin behaves, and it relaxes once it's on and moving.
When you're home, wash everything cold on a gentle cycle and either tumble dry low or hang it, and skip the fabric softener since it coats the weave and slows the breathability down. Sizes run newborn through 4 years, so it's worth checking the size chart before a big trip rather than guessing. If something doesn't fit right, Epic ships worldwide with tracked delivery and takes unworn returns within 30 days. We're Vanessa and Bob, we design these in Bali, and we've packed plenty of bags with exactly this question in mind.