Looking for an idea of what to buy

I am in the middle of redoing our entire porch. My next step is to put my railings up. Our porch is 26 feet long and 8 feet wide with steps at one end. My wife wants to keep it boxed in so our young kids can play out there.

Between the posts there used to be just osb. Facing the street we are putting fake wood siding type stuff but it has to attach to something solid. On the inside my wife wants it to be a solid like material similar to drywall which she then wants to use molding or something to make picture frame like designs.

Hopefully that made sense, so what I am looking for is something like drywall that can be outside. The porch is covered and this is one the inside so it won't be in direct contact with water but it would get wet from blowing snow and windy rain storms.

Someone told me to look at cement board but when I google that it just came up with drywall. Can someone point me to a product that might work for this project?