Target has inexpensive polyester long underwear for kids along with kids’ long-sleeve polyester athletic shirts (both Champion brand). And as of today, they also had Champion polyester athletic tops & bottoms in Men’s sizes on sale for $ 5-7 a piece.
Academy has BCG brand polyester leggings & shirts for about $10-15. (Think: Under Armour, only not nearly as expensive!) I’ve also heard another place to find reasonably-priced polyester underlayers & wool socks is in the hunting section at Walmart.
In cold weather, layers of synthetic fabric (polyester, nylon, fleece, etc.) with a wind-proof outer layer & wool socks is the way to go, especially if you will be working up a sweat doing things like skiing or backpacking. As the saying goes, “Cotton kills” – it absorbs moisture, is slow to dry, and chafes when wet. In short, it increases your chances of hypothermia and general discomfort.
So, it pays to buy polyester/nylon & fleece for outdoor activities. And the kids will use them again and again on all sorts of trips!