Decks, Patios, Porches and Docks - Structural Question for Child's Playhouse

I want to build an elevated playhouse for the kids. The deck height would be 7' off the ground, and the platform would be 8' x 8'. I want to support it only at the four corners if at all possible.

I had planned on using a 4x4x12' post at each of the four corners, and 2"x6" stringers around the bottom at ground level, and at the 7' deck height to hang the joists, and 2x4's about 30" off the deck elevation to attach the balusters.

My questions:

Are 4x4 posts sufficient or do I have to use 6x6?

Are the 2x6 stringers heavy enough or do I need something bigger?

Any other suggestions to make the project structurally safer?

Thanks in advance for your insights.

I would think that the 4/4 is sufficient. I would add diagonal bracing though You seem to be losing a playroom under the deck. Why not make a room.

Joneq has one request. When you build your playhouse please consider the neighbors and how it will affect their quality of life. Build it somewhere to least affect the neighbors New toys bring unwanted noise to the neighbors. Quiet is good noise is bad. Believe me I know. That is one reason joneq can get cranky. Constant noise from the "trampoline next door. :wall: :wall:. I hate summer because of it.

This assumes you have neighbors and haven't already taken this into consideration