How much ingredients for this cake?

Hello everyone,

I am extremely new to the cake business and I have my first order of my own daughters 1st birthday. I am looking to create a 6 tier multi layer rainbow cake for everyone. We have about 45 guest invited to the birthday party. Now I am thinking if I do a 10 inch and a 8 inch round cakes (both 6 layer) that should be enough.
So couple of questions:
- How much batter would I need?
- How much frosting would I need
- Also since this is outdoors I need recommendations on a frosting that will be loved by kids and can stand its base without melting in the summer heat.

Thank you in advance.
Ishak