Thank you for your Question.
Many children can go through fear. And at times it can be for unknown reason.
So you may want to speak to her and find out what really bothers her and make changes accordingly. Also you may want to play Quran in their room until they all fall asleep.
Another helpful measure would be to put her to bed a little earlier than her siblings so that she is fast asleep before everyone and her mind is at peace.
One last thing to do would be to offer sleep enhancing drinks, like slightly hot milk with honey. This brings a lot of peace to mind and the child (or even an adult) will sleep better.
Chamomile tea has been very useful too, but maybe more suitable for adults.
Also, it is highly known that if you write down the names of the Ashab al-Kahf on a piece of paper and place it under the child’s pillow then the child should sleep through InshaAllah.
I hope the problem is resolved.