There are a couple of posts above about monitoring internet usage. To add a different spin on this:
Some parents are comfortable with monitoring everything their child does on the internet, others aren't. Neither is perfect parenting and neither is wrong. Arguably, our most important job as parents is to protect our children while equipping them with the skills for adulthood. How we do it will be very personal to our own parenting approach e.g. for example some may set technology parameters while respecting their child's privacy and teaching them about critical thinking, others may monitor all use. Some may even stop their internet usage all together (that's how Erin Friday chose to approach things when her daughter declared a trans identity) in certain circumstances, others may think this in itself would be detrimental to how their child engages with the world.
The influence of the internet in helping children and young people believe that they are trans, when in fact they may be confused/distressed about their changing bodies during puberty or showing a trauma response after sexual assault, is massive and shouldn't be underestimated. But equally, there is another risk that a child will find their parent overbearing or will lose trust in them. There is a lot of info out there that says if a parent is not affirming your new identity, they are a threat to you and you should go no contact.
Another way to approach it could be to assume your daughter is accessing all of this information and that there are people in contact with her who are encouraging her to view a trans identity as the right answer for her. Some people on these forums may suggest a cautious approach (e.g. starting with "just pronouns" to see if that helps) but invariably the advice is the same: if that didn't help, why not try the next step (e.g. breast/chest binding)....? And so on, along the affirmation pathway.
I spent quite a bit of time on the same forums my daughter was likely to be accessing to see what was being said. This was back when I had no idea that there were places I could go to for information on it from a different viewpoint. It really is everywhere, not just the dark corners of the internet like Reddit e.g. the NPCC has a chat forum where children are sharing advice on how to access "gender affirming care".
IMO the most important things are to a) mitigate the impact of this bias as much as possible but without compromising your own parenting values and b) prioritise your daughter's mental health. She has clearly been through a huge trauma with this sexual assault. As part of prioritising her mental health, in addition to trauma support which is neutral on the subject of gender identity, maintaining your relationship with her and her trust in you is paramount.
Also, I echo RedToothBrush's caution above about Mermaids. I'd give them a wide berth, personally. Excellent advice above from PriOn1 too.