Many people, when they say 'culture' in the context of kids, mean something that kids can discuss around a lunch table. If OP's kids don't watch youtube, they won't have this particular aspect of culture as an inroad to make friends.
>>johnfn+(OP)
Like other humans, kids don't have universally aligned interest in media. Also, "missing out" can be good in many cases, depending on the content of said media.
>>someot+Zk
Sure, but if YouTube makes up say 20% of culture, that's 20% of conversations they cannot participate in. I'd love to read any source that says that "missing out" on making friends is actually a good thing.