Always nice to see the Otherkin pop up again nice break from the transexual madness.

So basically if she's self identifying as a Therian, it means she's gone down some internet rabbit hole and isn't mature enough to separate online crazy from reality. It also means she's probably gotten into weird chat rooms and those are filled with pedophiles ready to play the "your parents don't understand you but I do!" Pied Piper game and the kid will follow if you aren't careful. The younger child is probably just copy cat behavioring so it isn't an issue.

For the older child what you do is you go log into your internet router and start monitoring their internet history. Figure out what they are doing or where they are going. If they aren't going anywhere or talking to anyone weird, it's probably just a bizzare teenage phase they'll grow out of presently. Figure out if it's trending with her friends by talking with other parents to figure out if this is just a weird teenage social fad or if it's something she's getting from the dark places of the internet. If it's just cringe teenage stuff then it's often best to let them figure out shit on their own provided it doesn't go too far.

If it's dark internet shit then you have to be careful, being too heavy handed may push your child away directly into the predators waiting jaws, gather information first by keeping tabs on their browsing history but don't let them know that. Maybe block some IP ranges on the router so it looks like the sites they use don't exist anymore. As for the smartphone better to never give those to young kids in the first place but once the genie is out of the bottle it's not going back in, my solution, make the kids pay for their own phone. Yeah I could put you on the family plan but I don't feel like paying for your data. You pay me for it I'll put you on. Time to maybe go get a job? If you're paying for your own internet then you're out working in the real world and hopefully not getting sucked into the online space fully.