Building a playhouse - Part One

We have two little ones: W, a boy, is coming up on three and C, a girl, is coming up on one. I want to build a playhouse! I've browsed the plans (both free and for purchase) on the web and am unimpressed. Here are the attributes I'm looking for:

It's not going to be a treehouse, so there needs to be some element of climbability. So why not make it a flat roof? With a ladder up from the inside through a trap door! Terrific. Need to make sure the trap door won't slam shut on small fingers. Need a railing around the roof/deck. Let's add a slide off the roof - create an opening in the railing. I'm also thinking a rope net would be fun for climbing up. How about some rock climbing grips up one wall? This is going to be fun!