The phrase "thing adults" refers to activities, responsibilities, or behaviors that are characteristic of grown-ups or mature individuals. Full definition
Either you are an Adult who does things Adults do, or you aren't.. (cowboymate.com)
There's a full - on atheist in the film; her atheism is treated as a local secret, the kind of thing the adults know but children must be kept from hearing. (firstthings.com)
The most important thing any adult in schools can do is to envision students as partners, and then act that way. (soundout.org)