I need some advice on what materials or process to use when constructing a 6'x6' platform base for a simple kid's playhouse. I want the platform so that the playhouse interior is not just exposed to the ground (dirt & grass). I live in Michigan so the playhouse will be exposed to a lot of rain/snow/damp soil/bugs etc. - you name it.

My plan is to use pressure treated lumber where the platform contacts the ground and top it with a plywood cover.

My concern is that the underside of the plywood top will be exposed to ground moisture and bugs from underneath. Can you recommend if I should use ground coverings under the platform to minimize ground moisture? Should I use pressure treated plywood?

Thanks!