Let's compare the words spanked and hit.
Hit can come off as a very negative and violent connotation. Per internet, the definition clearly states hit being an act of quick force.
When you say you hit a child, that specific verb choice makes everyone think that you hit the child in violent and mean intentions.
Spanked is a term which is commonly used when a parent hits a child in a scolding manner. As a definition, it is usually only used when explaining parents hitting a child to scold; for example, a parent may hit a child in the hand after seeing him kick an animal on the street. This is a form of hitting that is scolding.