The TVTropes Trope Finder is where you can come to ask questions like "Do we have this one?" and "What's the trope about...?" Trying to rediscover a long lost show or other medium but need a little help? Head to Media Finder and try your luck there. Want to propose a new trope? You should be over at You Know, That Thing Where.
I feel like that would be a subtype of No Social Skills. I'd actually be interested in knowing if there's a trope for that, actually—my father, as a horror movie fan, did something like that to me when I was little.
For that specific example, it could be a downplayed version of Harmful to Minors, possibly an in-universe case of What Do You Mean, It's Not for Kids? Can also result in becoming an honorary Cool Uncle if the kids like the stuff they're not supposed to do, and that the non-parent's cluelessness lets them get away with. Can also play into Adults Are Useless.
Can't think of anything closer, though. "Clueless Around Kids" does seem tropable if we don't have it.
There is also Innocently Insensitive.
Is there a trope for a character who doesn't dislike kids but doesn't know how to interact with them? An example would be a character not understanding that playing a violent movie in front of a child isn't a good thing but would do so in an attempt to bond with the child.