I don't think freelance influencers care about their target audience, it's a task for the advertiser not the blog. Influencers are focused on maintaining the blog, to keep it updated. They need money to run the blog efficiently. Advertisers like casinos, offer them enough funds to place those ads. Because they, the casino, see the blogger discuss gambling in his online space. Most gambling ads are placed on game related blogs. Else the conversion rate will be low. However, advertisers may not care about the niche, if the influencer has enormous number of followers.
Such ads are disseminated across such pages, because out of those millions of followers, they'll be thousands of gamblers. Who will be interested in trying out the advertised gambling platform. Moreover, in the context of this thread, advertisers may want to convert non gamblers in the page to gamblers, by placing such ads aggressively. That is the influencer has to promote gambling on every single content he creates. Hence, kids visiting such page will one day get interested in gambling. Especially if the ads are spiced with a cash prize. Internet users are mainly tricked with giveaways and related ideas. "Money call the shots or makes the mare go", so, the influencers can't resist some cash offers for an ad placement, even when they dislike placing such ads.
I think that every influencer likely knows their target audience quite well. Given that influencers have built up a large following by consistently producing content that engages their audience, they should naturally have a good understanding of what resonates with their followers.
Not sure if you're aware of the aggressive advertising technique casinos implement on the internet. The advertiser at some point understands that gambling is a broad niche and lots of people will definitely get interested in gambling once money is flashed on their computer screen. Hence, the influencer may be away of the followers on their blog, yet it doesn't change the fact that they'll get over such thoughts when money is offered to them. Influencers are well aware of underage followers viewing the page, but that's not the aim of the project. Money is the aim and objective behind the building of the account. Not some kids viewing the page.
Casinos also cares about the niche the blog is built under, to some extent, but not when the budget is wide, and the house needs lots of outreach on the web. It's a disadvantage to the kids and society, and a huge win for the casino. In this deal, the consumer's age is not what matters, the casino assumes everyone knows gambling is not for underaged players. But the young people also don't care about this, I think the negligence is on every internet user involved in the ad distribution circle. Those kids also have no self-control, that's why they go after a game not meant for kid, for money's sake. Everybody needs the money, the casino, influencers and kids.