Baby Blues by Rick Kirkman and Jerry Scott for August 02, 1991

They bring this up in a later strip: proximity to the mom increases the pain (or something like that)

My brothers would clunk their heads on something, and wouldn’t cry unless you made a fuss over it. If you stayed calm, they’d just carry on.

Haha! (＾▽＾)

Quino made the same joke in a Mafalda strip about 20 years before this one.

There is a C&H strip about this.