When to use "nails" vs "screws"?

Good day,

Thank you for your question. That is awesome that you are making a birdhouse with your daughter.
You can use both nails or screws when making the structure. It all depends what tools you have.
A nail is a metal spike with a broadened flat head and smooth shaft that is driven in wood by means of a hammer. Therefore, if the only tool you have is a hammer, I would suggest using nails for the project. 
A screw is also a metal spike, but the shaft has a thread running around it. In addition, the head of the screw is slotted to allow a drill bit to fit into it. There are two ways a screw can be inserted. A hole can first be drilled into the wood and the screw turned in by hand with a screwdriver or with an impact driver. Or, you can use an impact driver to drive the screw into the wood. 

I hope that this explanation helps you.