the disastrous YouTube monetisation policy comes in part from a desire to keep the site “child friendly”

Sure, but the reason why they want to keep the site “child friendly” is because content for children is incredibly profitable and because advertisers don't want their ads getting related to "controversial" content.

Even if YouTube was run by a worker co-op, or was a state enterprise those two factors would likely still lead to stringent monetisation rules.

This is the reason why I don't like equating socialism with "workers owning the means of production". Worker-cooperatives can exist in a capitalist economy, which means they have to follow capitalist rules (including the drive to generate profits).

When leftists say "workers", they generally mean "the 99%" or "the working class", not individual workers. When leftists say "the means of production", they mean the economy/industry overall, not individual companies.

If youtube was owned and operated in common, it would not be bound to profitability, but to use.

We can also look at something like peertube, which is essentially a commonly owned version of youtube. Instead of being guided by profitability, it is used based on many different use-cases. There can be peertube instances that are completely private, there can be peertube instances that are used for a specific topic or community (for example kids) and there can be peertube instances which are not for children at all.