Is it permissible for a wife and husband to be partially naked infront of their child for the sole purpose of teaching the the physical difference between male and female or we should be showing medical diagrams?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

If this part of the body is not permissible to show to him, then it is forbidden to expose it to him, such as the 'Awrah (parts of the body that must be covered in Islam), and the like.

If it is a body part that is permissible to show to him, then this is acceptable. However, there is no need for this as there are teaching aids available in libraries through which you can teach him, among which are the medical diagrams you mentioned.

Allah knows best.