noun, noun or participle which takes 'suru', transitive verb
showing each other; comparing by showing
Casual reciprocal action: two or more people show something to each other, often in a playful or competitive way. Commonly used as 見せっこする.
子どもたちがおもちゃを見せっこしている。
The children are showing each other their toys.
テストの点数を見せっこしよう。
Let's show each other our test scores.
見せ合う is a more standard verb for 'show each other', while 見せっこ is a casual, often childish noun/suru-verb form.
Derived from the verb 見せる (to show) plus the reciprocal suffix っこ, which indicates doing something mutually or competitively.