I know how you feel, my son was transferred to Infectious Diseases Ward, around your niece's age. I stayed with him, as he was hospitalised for some time, and we were with other children, with an infectious virus, all were alright (I hope this helps you).
The Doctors may carry out some tests; support of family is so important; they have you!
I will pray for you all.
Please keep us updated, and if you need to contact me, please don't hesitate, you are most welcome to!