They cannot have their cake and eat it too.
If Link (according to Miyamoto, 12 in 1986), or Little Mac (17 as of the first game release 1987), or Sonic (15 years old when first game released 1991)... are considered children, what is ~30 years after their debut... Then they have an issue in general. Probably a lack of grey matter.
The opposite is what you discuss. The visual element being under an expectation of an age but the mental age being adult+. Regardless it is the physical body that is being sexualized and that is considered underage porn.
And all this only applies if you believe that cartoon characters and other fictional beings are able to have rights. Of which they cannot.
For the sake of saying it, just because it comes up regardless, it doesnt matter if it is legal. A website has the authority to prohibit anything it wants to. Regardless of legality of that content. So long as they do not prohibit something mandated to be required by law, the website is conducting legally. So while "shota of cartoons" may be legal in a lawful sense, the site can still prohibit it and the site' actions there are legal.