Calculate Incidence rate for disease with fertility/mortality data

I am trying to calculate disease incidence in a certain country. I have the number of cases detected over 10 year period. The disease affects children mostly age 5 and younger, but my inclusion criteria are for children <10 years. I also have the country's fertility rates and child mortality <5 years (in 5 years increments) from UN development statistics.

How do I calculate incidence rate, either per 10,000 or per person years. Do I need additional information? I'm assuming minimal population migration.

Thank you for the help.
Do all of your data come from the same source, or is it piecemeal.
Thank you for your response.

It's sort of piecemeal.

My diseased cases come from one source (two clinics in the country see these sorts of cases, I have 33 cases from my clinic and I know the other clinic has seen 9-10 cases), so I was planning to consider this as part of a correction for cases.
In general cases are referred to my or the other clinic, or for those patients with more $$ they may seek care in another country (I dont know how to estimate this, would consider it a limitation to the study).

I have mortality/fertility data for the time period from one source (UN population data, which is what UNICEF also uses...

How does one consider differing sources change the calculation?