I hope that someone with a medical background is able to give you specific information. You are leaving in a few days so I will attempt to give you general advice. I have visited several medical clinics in rural Nicaragua. What I have found is that the clinics have a shortage of all medical supplies.
Technically, you are supposed to get permission from the Nicaraguan Ministry of Health to bring medical supplies into the country with the exception of supplies for personal use. If you are carrying a small amount of supplies with your belongings you will be waved through. If you are carrying a suitcase full of supplies customs officials with check the contents. Do you speak Spanish? Custom offices do not speak English. If you can explain that the items are for a rural clinic they may let them through without permission. It would depend on the agent. You should also carry an itemized list in Spanish of the contents of the suitcase.
Are you spending any time in Managua before flying to Little Corn? If so, you can find all the medical supplies you need in Managua. Most medical supplies would also be cheaper in Nicaragua. However, you would likely find better quality in the United States. You can also buy without a prescription all types of medicines in Nicaragua with the exception of narcotics.
I know that this does not answer your question specifically but I hope I was able to give you some guidance. Your donations will definitely be welcome.