I use the same construction practices as you would expect any high-end general contractor to use on your own home. They start with doug fir 2x4s and plywood sheathing, and if you were to look inside the walls you'll likely find R-13 insulation and 12-2 wiring. Structurally, they are just like your house.
Point is, Magical Playhouses are built to last. Not only for environmental reasons, but because I want you to feel like what you bought was worth it, and worth is a function of time. And whether it is still there in your backyard is the difference between using 8d framing nails on your shakes, or doing the right thing and using the more expensive stainless steel 305 ring shank nails. You'll never see either when you buy it, but especially around Seattle, in our slightly salty marine air, you'd know the difference in about eight years.
Let me give you another example. See the blue stuff in the picture on the left? That's rigid insulation. I put that in because that space gets covered up by a nailer during framing, making it impossible to stuff batt insulation in there when the time comes. Honestly, you probably would notice that the playhouse had slightly colder corners if I didn't do it. But I do it because its the right thing to do.
There was a banner over our workspace at carpentry school that read, "Craftsmanship is integrity you can touch." I believe that. It's why I pony up for those stainless nails. And if it's for your child, it's why you should expect your Magical Playhouse to outlast the expiration date of every car seat they will ever use.