Another idea that has been successful for our kiddo is having a TV on playing an age-appropriate content. This isn’t for all kids, but it does help some children. You can also try a tonies box, which is more of an audio option. Another option is you can try taking a step back from the idea of sleeping in her own bed and just focus on sleep. Sleep can occur anywhere like family living room camping as an example.
This happened to my sister and my parents told her monsters hate her dad’s underwear and they would put pairs of my dads underwear around her bed. I’m not saying it’s foolproof but worth a shot.
Perhaps try distraction that’s not a bright light like a projection night light and screen free listening like a bedtime podcast, Yoto player, white noise, etc until she falls asleep then turn everything off for her. And teach her if she wakes to try turning the audio back on to soothe herself. It’s sounds like it’s going to be a short term phase def caused by whatever she saw with dad unfortunately. Is it also possible to have big sis help her feel safer by swapping beds for awhile?
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Get her a monster repellent spray bottle! 😉
I'd try giving her more power in the situation. Does she have any fluffy stuffed animals that she likes? You can say that toy has nightmares and needs her to be brave and protect it from monsters. That switches the roles around and gives her agency: she gets to be the one in the calm protector role.
A lamp can trigger flight or fight at night and cause you to stay in very light sleep meaning once having a nightmare you are awoken by a bright light on at 2 am can all be very scary to her. When my kids have watched something scary you just have to wait it out a week or two and keep reinforcing the nightlight and letting her know her big sister is there and she is safe. Try to Keep a consistent routine so her body feels regulated at night. I had to talk to my husband and get on the same page about things to watch and not because we have been over this time and time again. He finally realized and stopped and no issues since