Is a child's crown usually made of stainless steel or porcelain?

Depends. Baby teeth are generally covered with simple steel crowns unless the parents are ready to spring for the higher cost cosmetic crowns.Preserving the baby teeth saves spacing & room in the gum line for the permanent teeth.After the permanent teeth erupt, the choice of any crown will depend or your wishes(& finances).
Stainless steel . When a primary ("baby") tooth needs a crown restoration, we usually use the prefab. Stainless steel crown because in a few months the permanent/adult tooth will replace it.