There is a precedent for regulating this sort of thing. Advertising to children is regulated to some form or another just about everywhere.
What's new and challenging here is that when a person simply posts a video they made on YouTube, that's freedom of expression. If I want to post videos of me unboxing things, there's nothing morally or legally wrong with that.
The grey zone is the money being traded between toy makers and video makers. As soon as money is exchanged, that should logically be considered sponsorship and be subjected to advertising laws.
Just because it's on YouTube doesn't change that a company is paying money to an entertainer to promote their products to children. There are already laws in place about this. It's only a matter of time before prosecutors get involved.
Of course, the toy companies know this and have factored in the legal risks into their calculations. YouTube will be able to skirt the issue because they're just a platform for other people's stuff, and they're not being paid for these sponsorships. The real risk is being taken by the content producers, who almost certainly don't realize how liable they may find themselves to be.