How to learn about children using hitting affectionately
The search term "children hitting for fun" seems to yield the results you may want although the results seem to come from sites such as mumsnet.

An aspect to this phenomenon is that this behaviour may lead to bullying behaviour later on in life if the right responses are not given when this occurs.  The child needs to learn the cause and effect of hitting.

Typing the same term in Google Scholar gives research articles on dealing with bullying.