You gotta use childish abstractions and cartoons to maintain the illusion of gender identity. Once you graduate from unrealistic forms to photographic depictions of trans people the difference is fairly obvious in most cases. It's basically why trans circles are so rife with anime content. It's not so much that anime content is trans in nature (though the differing cultural standards from a society with a more open and matter of fact approach to sex, and stricter gender rolls than the west, do tend to have fictional depictions that cross what in the west would be considered taboo and age inappropriate, most is innocent in the same sense that, bugs bunny crossdressing to trick Elmer Fudd is a joke not meant to be taken seriously) it's more that the anime girl phenomenon represents a largely male idealization of the female form, and naturally that's the idealized fictional identity that is adopted as being unrealistic and exaggerated allows for the illusion to be maintained in online spaces whereas photo realistic depictions quickly betray the birth gender of the person in question in most circumstances. A similar phenomenon exists with women as well in the form of Yaoi popularization where women create these strange idealized male forms from a female perspective that aren't at all indicative of actual male relationships.

For the most part the best thing to do is keep your children away from unrestricted and unsupervised internet access. You can't stop them completely but merely slowing their access and asking what are you doing will oftentimes be enough to either cause them to desist from problematic internet usage or allow you to identify what they are struggling with before they can be influenced by creepy strangers online. Smartphones are probably ok to let your kids use only if you are clear that nothing they do on the phone is private and you can see everything, and then you've got to monitor it. Unless they are willing to pay for the phone bill themselves, that's the condition of use. Teaches them to be careful as in real life you also can't expect your electronic communications to be private by any means.