I like these:

https://www.amazon.com/gp/product/B072M1RT47/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

I get all our kids sheets at Home Goods, but you have to read the label. There's a separate kid section with the kids sheets. I'd say half are all cotton, half are a poly blend. In the regular sheet section, all the Tommy Hilfiger sheets are polyester, so don't be fooled by the cute patterns.

We recently splurged on Lands End but I often have good luck at Home Goods or similar stores. I agree with PP, you just have to read the labels carefully. Sometimes the front packaging says "cotton" but the small print says it's a blend.

IKEA! I also like their waterproof mattress covers.

I love Ikea sheets.

Target has them. You just have to check carefully on the label.

Ikea, different price points for different levels of quality. The cheapest ones are REALLY rough and scratchy though

I bought sheets from target and kicked myself after washing because I realized they are poly. Do you have twin/full size cotton sheets you love for kids? Also looking for recommendations for a waterproof mattress cover.