If I am a school nurse and am having issues with a 4th grade student and her parents, is is legal for me to use a voice recorder when this student visits my office?

It may be quite illegal especially if you live in a state that requires consent from the parties (all of them) being recorded. Futher, you are also talking about a minor so there is a lot to deal with legally concerning a minor child being recorded without the consent of the parent(s) or guardian(s). You have to mitigate harm to you so you may consider speaking with and confirming in writing your conversation with your supervisor. The conversation will help put on the table all of the problems you have with this student or the parents. If it is abuse you suspect, you are a mandated reporter and must report it to child protective services. If it is other issues, speak to the main superintendent or deputy superintendent of schools about the issues you are having and what the school district can do to step in and help.