Send a certified letter, a copy of the same letter in the regular mail, and even a hand-carried letter that you go back to the office with TODAY (promptness is key since the office is closing) requesting your children's ENTIRE record, (this would include any x-rays that may have been taken also).
Which local hospital was she associated with? Contact their freedom of information office/records section and ask them for help too.
Did your doctor have a website or email? Contact her that way also.
I have blank forms from my doctor that I am willing to scan and email to you if you need examples.
If you receive no response in a week, contact the local medical boards.
This will all take time, but is well worth it to have your childs entire medical record to have to give to your next pediatrician.
Good luck.