Do I calculate my NZ dollars into pounds before entering?
They don't care as long as you state on the form which currency you are using, looking at the form's instructions...
If you do decide to convert the amounts to Sterling, they like it if you use Oanda spot rate because it's the one they are accustomed to. But you should always explain which rate and market source you are using in the remarks section.
Do I have to fill in another visa application for my son, 2 years old?
Yes, even if the child is 1 day old, you need to submit an application for them. If you are applying as a family, you do not have to duplicate all the evidence for the child. Your own evidence is sufficient.
Also we might be flying via LA. Do i need a transit visa for America
This depends upon your nationality. On this site we use Do I need a US visa to transit (or layover) through an American airport? as a canonical way to answer questions about transit visas for the US.
There's some general information about how to make the process smoother using the parent's GWF number here: How should a family with young children apply for UK visas?